Why not Charleston, BTW? Especially if you look at the historical populations of the cities we're talking about, around the time of the POD. The wealthiest city in the Thirteen Colonies, and the largest in population south of Philadelphia, as well as the 4th-largest port in the colonies, after Boston, New York, and Philadelphia at the declaration of independence (officially- in reality, the USA's 3rd-largest port in the first US census in 1790, comfortably above Boston, after one takes into account Charleston's majority slave population, given the fact that said census only counted slaves as 3/5ths of a person up until the revoking of the Three-Fifth's Compromise after the US Civil War), keeping the lead over New Orleans as the largest city in the Deep South until 1820 (at least- with Charleston in the 1820 census having reported an official population of 14,127 blacks and 10,653 whites; after taking the 3/5ths rule into account though, if slaves had been counted equally, it'd have had more like 22-23,000 blacks, making its total population narrowly larger than New Orleans' at least until the 1830 census). But the purpose of this AHC is to make it larger than New York; and geographically, Charleston Harbor's ideally situated to be a Deep Southern alternative for Hudson Bay as a major entrepot for transatlantic immigrants to the USA.
IOTL, Charleston built its economy and prosperity almost entirely upon the backs of slavery; which worked extremely well for its newly self-established aristocracy, in the short term, but worked to the detriment of the rest of the city's population and the expansion of the city itself in the long term, deeply entrenching an oligarchic society controlled by about ninety interrelated families, where 4% of the free population controlled half of the wealth, and the lower half of the free population- unable to compete with owned or rented slave labor- held no wealth at all, with Charleston continuing to remain the only major American city with a majority-slave (and needless to say, majority black) population throughout the Antebellum Period. The 'middle class' barely existed there- with Charlestonians generally disparaging hard work as "the lot of slaves". All the slaveholders, taken together, held 82% of the city's wealth, and almost all non-slaveholders were poor. Olmsted considered their civic elections "entirely contests of money and personal influence" with the oligarchs dominating civic planning; the lack of public parks and amenities was noted, as was the abundance of private gardens in the wealthy's walled estates.
New York City, IOTL, attracted so many people because it rapidly became viewed globally as the city which offered the greatest opportunities, the gateway to the land of opportunity; between 1820 and 1920, 35 million immigrants arrived at U.S. ports, with 82% of these arriving at the port of New York, and a good portion of these never left, with four fifths of New York City's population being foreign-born by 1910. Charleston, however, whilst it had been one of the five largest and most populous cities of the USA at the time of the earliest POD(s) for this AHC, rapidly fell into relative population decline, dropping out of the list of the USA's 10 largest cities after 1840 never to return, thanks to its population largely stagnating and with no immigrants heading there at all (excluding the human trafficking of imported slave labor, from Africa and from other eastern coastal areas of the USA, by its oligarchy and by smugglers), because it rapidly became acknowledged as a city which offered no opportunities whatsoever; ruled over by the slave-owning oligarchs who'd 'pulled up the ladder' behind them as quickly as they could, in order to stymie any newcomers' efforts to eke out a living and earn their own way.
In essence, socially, societally and governmentally, Charleston was the closest RL equivalent to Gotham City; ruled over by the closest RL equivalent of Gotham's 'Court of Owls', a violent cabal of the city's oldest and wealthiest families who used slavery, murder and money to wield absolute political control over the city (and by extension, via their dominance of the domestic slave trade, over the entire Atlantic seaboard of the Deep South, along with the city's hinterland, which later developed into the 'Black Belt') and presided over it from their walled estates, hoarding every bit of wealth they could for themselves to the detriment of the city and the wider region. Unlike Gotham City though, since people could choose which US port they wanted to immigrate to the USA through, anyone who wanted to 'make it' in the USA went out of their way to avoid moving there, stymieing its further expansion. Unlike fictional Gotham's 'Court of Owls', who presumably wanted to maintain the influx of their underclass of European immigrant plebs, but who were willing to pay them and maintain the illusion of their freedoms and equality whilst keeping themselves in power from the shadows of the night, Charleston's own 'Court of Pineapples' openly basked in the full glare of the sunlight; discontent with merely ruling over a population of mere peasants, with equal rights, freedoms and opportunities to themselves guaranteed by the US constitution even if they were only on paper as Gotham's 'Court of Owls' did, they actively dissuaded any lower-class white migrants from entering the city, simply expanding their own underclass of black slaves to serve as human livestock for them instead, and would likely have sneered at Gotham's 'Court of Owls' as weak-willed libertarian cowards.
Also worth mentioning, akin to Gotham with its 'Slaughter Swamp', Charleston also developed a reputation (encouraged by its slave-owning oligarchy, first establishing the 'scientific argument' which was subsequently co-opted and used as one of the primary justifications for the practice of 'blackbirding' in many other places around the world from that point on) as one of the least healthy locations in the Thirteen Colonies for ethnic Europeans, rendering it 'unsuitable' for greater European settlement and 'necessitating' the use of non-white labor instead. Malaria was endemic there, and remained a major health problem through most of the city's history before dying out in the 1950s after use of pesticides cut down on the mosquitoes that transmitted it. So, how can we prevent this from happening, then- how can we get Charleston to avoid its decline, and overtake NYC as the larger city? The obvious first necessity is to get rid of its cabal; remove the 'Court of Pineapples' strangehold, and the opportunities open up for the city, as they do for the wider region as a whole. Getting slavery abolished, and fast-tracking the civil rights movement, would also greatly improve Charleston's chances; half of the US African-American population entered the USA via the city, but its African-American population is actually lower today than it was 200yrs ago. My proposed POD, to intitate this? Have
Denmark Vesey's planned uprising succeed, at least up to a point.
IOTL, it was foiled thanks to the testimony of two slaves who came forward to inform the city's officials a month or so prior to the revised date when it was planned to go ahead, on the 16th June 1822; Joe LaRoche, and George Wilson. IOTL, Joe LaRoche had originally planned to support the uprising, and brought the slave Rolla Bennett (one of Vesey's four main lieutenants, and the primary house-servant of the then Governor of South Carolina, and wealthy entrepreneur, Thomas Bennett- with Roall Bennett later testifying that while "the governor treats me like a son," he was nonetheless willing to murder the governor and his family, for the sake of freedom) to discuss plans with George Wilson, his close friend. Ultimately, IOTL, Wilson refused to join the conspiracy, urging both Laroche and Bennett to end their involvement in the plans, and convincing LaRoche that they had to tell his master in order to prevent the conspiracy from being acted out. ITTL, though, let's say that George Wilson's either swayed to join Vesey's Uprising, sufficiently intimidated by Rolla Bennett to keep quiet about it, or that LaRoche simply delays bringing his friend Wilson in on their planned insurrection until it's too late for him to prevent it from happening.
And to improve their odds further, add in a powerful 'divine omen' aspect akin to Nat Turner's rebellion, and get another major event later on which would greatly impact the city's development later on IOTL out of the way, how about bringing the 1886 Charleston Earthquake forward by 64 years or so, to take place in early to mid-1882 instead? Charleston's earthquakes, as intra-plate earthquakes, are large but infrequent, now known to occur roughly once every 500–600 years; with OTL's 1886 earthquake registering at 6.9-7.3 on the Richter scale, and having been so severe that it caused speculation that the Florida peninsula had broken away from North America. The initial shock in Charleston lasted for about 45 seconds and was extremely destructive, leaving nearly all of the 8,000 city structures with either interior damage or broken windows, with the first aftershock following just ten minutes later, and at least seven different aftershocks felt in Charleston and its surrounding areas in the next 24hrs.
So, let's say that ITTL, this happens in May-June 1882, and as would have been the case, isn't reduced in intensity by that all that much (still having a projected strength of 87-90% that of OTL's 1886 Charleston Earthquake, keeping it at or above 7 on the Richter scale). The original Charleston Meeting Street Arsenal (demolished and rebuilt as a far more solidly constructed, heavily fortified building in 1829, as a reactionary move to the foiled 1822 slave revolt IOTL) is heavily damaged by the earthquake and aftershocks, and with Vesey and his lieutenants taking this as a divine signal and Act of God (sent by God to deliver them from slavery as he'd delivered the children of Israel from Egyptian slavery), they seize the god-given opportunity to storm it and begin their uprising slightly ahead of schedule, on the day of the earthquake (even in the event that the allegations of a vast city-encompassing insurrection plot were indeed largely overblown by Mayor Hamilton and the Court, who supported a more militant approach to controlling slaves, believing that the paternalistic approach of improving treatment of slaves, as promoted by moderate slaveholders such as Bennett, was a mistake; using the manufactured crisis to appeal to the legislature for laws which he had already supported,to authorize even more draconian restrictions of slaves and free blacks).
As such, ITTL, Vesey's Rebellion successfully seizes the weapons cache there with relative ease, before his followers proceed with their plans of emulating the Haitian example by killing the white slaveholding oligarchs of the 'Court of Pineapples', and liberating the city's majority slave population; as well as securing control over the harbor, with some commandeered ships from the harbor setting sail for Haiti (possibly with Haitian help, with records of the French Consulate in Charleston reporting that Vesey's group had numerous members who were "French Negroes," slaves brought from Saint-Domingue by refugee masters)- though this part of the plan ultimately fails to be that much more popular, or make more of a lasting impact, than the later colonization of Île-à-Vache did IOTL. And this uprising shakes the Deep South, and the Antebellum USA as a whole, to its core far more than the earthquake itself does; far eclipsing Nat Turner's rebellion IOTL (which only involved an uprising of 70 enslaved and free Blacks, limited to using knives, hatchets, axes, and blunt instruments by their inability to acquire muskets or firearms, supressed by overwhelming force within a couple of days- but still led to widespread paranoia and rumors about "armies" of enslaved people allegedly taking over North Carolina and Alabama for several weeks, and innumerable anti-Black pogroms), with over a thousand liberated slaves joining the revolt, which spreads across the entirety of the wider Lowcountry and Sea Islands region into a transformational conflict which rivals or surpasses the scale of the Yamasee War a hundred years earlier, with Jack Pritchard aka 'Gullah Jack' securing the full support of the Gullah people.
Early efforts by the USA's hastily assembled expeditionary militias to restore order to South Carolina, fare no better than Napoleon's Saint-Domingue expedition to retake Haiti a couple of decades prior; with Vesey's Rebellion having taken place right at the start of the summer months, when fevers, particularly malaria and yellow fever, ran rampant in the Lowcountry, outbreaks of these diseases among the US expeditionaries dispatched to the Sea Islands to try and subdue the Gullah communities inflict a similar or even heavier toll than they did upon the French expeditionaries in Haiti. And unlike with the Yamasee War though (which had nearly destroyed the colony, killing 7% of the settler population of South Carolina and killing or enslaving 25% of the Yamasee population, as well as forcing the abandonment of the Indian Slave Trade by South Carolina in its aftermath), where the tide was turned when the Native American alliance splintered, and the Cherokee switched sides to ally with the Colony of South Carolina and wage war against the Creek, there'd have been no such divisions among the Charleston slave rebels which could even come close to inclining them towards allegiance with the White slaveowners, and to assist the colonial, state and/or national militias dispatched to try and quell the rebellion's liberation of the plantations' slave population- save perhaps for one, and one alone. Namely, the promise of, or fast-tracked implementation of, emancipation.
As Virginia's legislators did with Nat Turner's rebellion IOTL, but earlier and to a far greater extent, emulated across almost all of the other states of the Deep South and in several other states as well (including New Jersey and New York, where the Colonization Society of the City of New York- established 5yrs prior to this POD- and the related New York State Colonization Society, IOTL played a pivotal role in providing the financial support needed to balance the books of the American Colonization Society IOTL), colonization bills are passed ITTL as a reactionary measure, offering to encourage and support the migration of free African Americans to the continent of Africa, along with police bills denying free Blacks trials by jury, and making any free Blacks convicted of a crime subject to sale into slavery and relocation. These would be received with even greater opposition, disdain, hatred and hostility by the overwhelming majority of black Americans than that with which they received the ACS's efforts IOTL though; and with the majority of the South Carolina Assembly members and its Governor having been in Charleston, and having been killed by the rebels to free their slaves at the outbreak of hostilities though, there's no time to enact these measures there. Having never had any intent of declaring independence from the USA, any more so than L'Ouverture and the Haitian rebels had from the French First Republic, but with its pre-existing legislature having been wiped out by the rebellion, and the first act of its new Governor appointed by the rebellion having been to declare the abolition of slavery in the state, any efforts to reimpose slavery upon the majority-black population there are doomed to failure, even if/when the USA quells the rebellion.
After what had happened to Louverture in the Haitian Revolution, and the violation by the French of their surrender terms (that slavery would not be restored in Saint-Domingue, that blacks could be officers in the French Army, and that the Haitian Army would be allowed to integrate into the French Army), the leaders of Carolinian Rebellion refuse to fall for the same trick; and if it becomes apparent that the USA intends to re-establish slavery there, it'll immediately invoke new revolts in the same manner. Instead, the likeliest course of action for the USA of that era is for it to reintegrate South Carolina by effectively setting it aside as a 'Black-only State', in a manner akin to the conception and implementation of Oklahoma/'Sequoyah' as a state for the Native Americans. And whilst significantly more people are forcibly sent to colonize Liberia ITTL than IOTL (with Liberia being significantly larger as a result, and Charleston already long pre-established as the primary entrepot for transatlantic maritime trade between the USA and Africa), this factor, in the aftermath of the end of the Carolinian Rebellion, results in an even greater 'Great Migration'. but from everywhere else in the USA TO the majority black free state ITTL, rather than the other way round. Charleston ITTL, as the city where Vesey's Revolution took place, the birthplace of the early American Civil Rights Movement (much like Atlanta eventually became IOTL, but well over a century earlier), and the First City of Freedom and Emancipation in TTL's USA, becomes established in the aftermath as the unquestioned US capital of the New South and as its sole 'Black Mecca', far eclipsing Atlanta and OTL's Harlem (which never develops into a black neighbourhood ITTL).
