Very interesting article. I remember first coming across Livy's speculation on what would have happened if Alexander the Great had lived for longer and tackled Rome when I was a Sixth Former (ie equivalent of modern educational Year 12) doing Classical History 'O' Level, and thinking that his conclusion was sheer Roman chauvinism. It was a political response ignoring the liklihood of one smallish mid Italian state defeating a huge empire run by a super-flexible military genius (albeit one who was often dangerously careless as to casualties and risks). At the time of the probable confrontation had A lived, which in my own imaginings as a teenager (made before I read Livy) I put at c. 314 BC once Alexander had led his Greek 'league' plus his Asian troops to destroy Carthage, Rome barely controlled the Campagna and Tuscany and at the most had a precarious grip on mid-Italy, and was loathed by equally strong tribal-based southern Italian states led by the Samnites (inland of Naples) with whom it was fighting on and off for 20 years. The main consular army of Rome was trapped in a mountain pass , the 'Caudine Forks', by the Samnites in 321 and was forced to surrender and pass under a yoke as humiliation - the Rome of the 310s was not the great power that it became after uniting Italy and defeating Carthage, and Livy of all people should have admitted that given that he had written 'official' histories of early Rome. His verdict seems to be Augustan-era propaganda and 'we are the greatest' spin, not logic.
On the issue of one man's life, ie Alexander's, making a major difference to the trajectory of Macedonian power, it is worth going back to the reported celebrations in M's foe Athens after Alexander's father Philip II, who had created the army and administrative back-up that Alexander used, was assassinated in summer 336. The Macedonians had routed Athens (and Thebes with its famous 'Sacred Band' regiment, allegedly of pairs of homosexual partners) at the battle of Chaeronea in 338 and forced it into the Macedonian-led 'League of Corinth' ready to invade the Persian Empire, so Demosthenes the anti-Macedonian orator and others assumed that killing off Philip would end the threat. One of the more canny Athenian moderates - Aeschines? - retorted that it would make no difference, except that the Macedonian army had been reduced by one man. In other words, the overall 'military machine' was what counted more than who was leading it. The same theory can be applied to Rome and its army - but not in the C4th BC.
The issue of a presenting a plausibly united 'military machine' under a stable leadership long-term and this mattering the most to creating a 'realistic' CF timeline is one of the keys to my Late Roman timelines . One of the main underlying themes is the question of 'what if the huge and , when led properly, effective Roman army had had no civil wars or coups at the time of its C4th and C5th challenges'? Instead, there is a reasonably stable run of leaders with few, instead of constant , coups, murders and other disruptions - thus we get a run of adequate or sometimes first-class leaders from Constantine 'the Great' to Julian to Theodosius to the OTL Western Empire leaders Stilicho, Constantius III, Aetius and Majorian. The result is a powerful and largely uninterrupted Western Empire military 'high command' able to dominate the weaker and more unstable German tribal leadership and create a long-term machine that can take over the East after 500 and later finish off Persia (and incorporate the Arabs). And in Greek/ Hellenistic history we can ask if a longer-lasting Alexander creating a stable military 'high command' and living until he has an adult son could have created at least a sort of 'combined Seleucid-Ptolemaic- Macedon' Middle Eastern/ Mediterranean state. This could have held off Rome - as in OTL the Seleucids under Antiochus III in the 190s and 180s BC tried to do . We end up with a 'Greek E Meditterranean' vs a 'Roman W Mediterranean', as a sort of 'Western vs Eastern Roman Empire' situation 500 years early?
One question which I also considered for my Roman timelines but which has universal potential - if there is a dominant and seemingly stable state with a united army that can outmatch its potential challengers, does the greater risk or hopelessness of challenging it (except if there is a civil war or incompetence at the centre) put off potential rebels and create a long-term momentum for unity? This is the key to my vision of how a longer-term Roman Empire could have worked as a sort of 'European/ Middle Eastern super-state', and if the 'Great Power' dominating a region also has a technologically superior and capable, flexible administrative apparatus it adds to its potential. I can see either a Hellenistic 'United Successor States' kingdom (Alexander's or possibly the Seleucids or pre-301 BC Antigonids) or a Byzantine Empire of Justinian's era - that was not hit by the 542 pandemic (now an apposite topic for commentary?) or economic exhaustion from the Gothic wars - doing this. The Western European Germanic states, even Charlemagne's, were probably too unsophisticated, institutionally weak, and dependant on feuding hereditary dynasts; the Arabs' C8th and early C9th Caliphate was probably too open to ideological/ tribal/ provincial military governor breakaways. But there's certainly a lot of CF timeline scope for creating a long-lasting European/ Middle Eastern equivalent of the Chinese cycle of 'collapsing militarily under external pressure or dynastic failure but surviving as a cultural entity and later being re-created' . Or if there's a longer and stronger Alexandrian/ Seleucid presence in the Indus region, eg a super-charged kingdom of Gandhara, then we could have a fusing Greco-Indian state under a capable dynasty creating a 'Successor'-style power that lasts longer than the short-lived Ganges/ Indus kingdom of Menander, adopts Indian cultural terminology but creates a Greco-Indian army, and adds a new layer to N Indian state development?
On the issue of political manifestoes wrapped up in CF fiction, there's also the question of Kingsley Amis' 'alt hist of a 1970s which has a surviving pre-Reformation Papal power in Europe' in his 1970s novel 'The Alteration'. Reading it at the time, I could see that he had mixed in elements of the (then very much a political bogey-man) USSR and its secret police with the Nazis and the real life Spanish Inquisition to create his dystopia of a papal secret police and bureaucracy dominating Europe - headed in his version of history by a cunning and cynical Yorkshire politician as Pope. (Some said that he had taken a swipe at then PM Harold Wilson, seen as clinging onto power by devious tactics and lacking ideological 'depth'.) And Amis' cardinals heading the Papal police and ministries included the then tabloid-feared Tony Benn as a sinister statist 'enforcer' targeting dissent, plus in the recent past Himmler and Stalin's sidekick Beria.