This is probably very niche, but I've been reading a lot about pro-republican forces in the Weimar Republic and it absolutely fascinates me, so here's a scenario.
Carlo Mierendorff was a German socialist, journalist, and politician in the 1920s and 30s. A young member of the up-and-coming generation of the SPD during the late years of the Weimar Republic, he had a talent for propaganda and a number of unorthodox ideas about what the party and the country needed. He was a dedicated republican but painfully aware of its flaws and deeply afraid that it might fall. Among other things, he supported revising the constitution, overhauling the federal system, and a move from proportional representation to majoritarian elections (probably FPTP). He helped co-found the Iron Front, a broad extraparliamentary alliance of pro-republican forces centered around Germany's powerful trade unions and the
Reichsbanner, which acted as the militant arm of the SPD.
Like nearly all socialists, he believed the Nazis were a clear and present threat to the republic and had many opinions about what could be done to stop them. As a propagandist, he focused on the public image and campaigns of the SPD and Iron Front, attempting to bring a new sense of comradery and activism to the fairly stale social-democratic scene. He sought to appeal to emotion rather than reason, as the SPD traditionally had, in order to counter the Nazis; among other things he helped created the Three Arrows and popularised the anti-fascist salute, a raised fist, among the Iron Front. Though his ideas were fairly popular among the membership, the SPD leadership thought they were cringe, and were hesitant to let him put them into practice. By the time he convinced them to try it on a larger scale, however, it was late 1932 and things were moving much too fast for his efforts to make any difference.
Essentially, in this scenario, Mierendorff's ideas are put into full force, including the sweeping constitutional reforms he advocated. There is no real string of events that leads here, because in my opinion the Republic was more or less doomed after ~1930, which is too early for a plausible POD. But basically the Nazis don't come to power, the Republic hangs on a bit longer, the SPD adopts Mierendorff's platform: radical reforms to the economy, constitutional, federal system, and electoral system, and manages to implement them one way or another (shameless handwaving). The Depression eases but the republic is still in peril.
Mierendorff believed that a majoritarian electoral system would force parties to focus on candidates and alliances rather than appealing to narrow demographics. He also believed that the Iron Front should be an active and enthusiastic bulwark of the republic, rather than a last resort option. In this scenario, it becomes a fully-fledged electoral vehicle for the republican movement, backed by the trade unions and energised youth. It comprises a broad alliance of the SPD; the United Workers' Electoral League (VAWB), which puts trade unions directly into electoral politics by endorsing or running candidates themselves; the Republican Centre League (RM-Bund), an alliance of republican liberals and centrists; and the SPD-L, the former SAPD which has been coopted to compete with the KPD for the radical left vote. Together, this broad platform storms to a near-majority in the 1936 election, the first held under the majoritarian system. Against all odds, the republicans have won.
But politics keeps going. The Iron Front was only able to win because of horrific vote splitting among the right-wing, who didn't quite get the memo about the new electoral system. Between them, the three largest right-wing forces (the Nazis, DNVP, and a bourgeois-conservative alliance) won almost 45% of the vote, but not even 200 seats. In the following years they toil to forge a national alliance to topple the Iron Front. Hitler staunchly refuses to join, because Hitler, but the remnants of the DNVP coalesces with various bourgeois, conservative, and national forces. The result is the National Front, comprising: the United National Party of Germany (VNPD), a broad-based national-conservative party taking in most of the DNVP, WP, and some members of the DVP and Zentrum; the DVP itself; and the Christian-Social People's Service, which serves to sharpen the alliance's appeal to Protestants. This alliance proves massively successful in the 1940 election, easily usurping the Nazis' place as the leading party of the right with 28% of the vote.
Just as Mierendorff predicted, Germany appears to have emerged into a two-party system by electoral necessity alone. There remain three significant minor parties, though: the Zentrum, which maintains a rock-solid presence thanks to its unshakeable base. It has its own alliance with the Bavarian People's Party (BVP) and small agrarian parties. Hitler's appeal had naturally waned after years of rhetoric and almost nothing to show for it, but suffered an almost mortal blow with the emergence of the National Front. The Nazis fall from second to fourth place, losing two-thirds of their seats thanks to the majoritarian system. The Iron Front's newfound energy and image damaged the KPD substantially, sapping them of much of their appeal, though they retain a clear presence in the Merseberg region, as well as Berlin and the Ruhr.