Les Evenements ft. Fucking Hell De Gaulle's Dead
This one, I think, has legs. Not an actual coup by the "Algérie française" movement (if I've seen it once, I have seen it a thousand times), but a successful assassination at the Petit-Clamart in August 1962 would probably kill the Republic as well. Not instantly but in slow motion, as no one can step into the shoes of de Gaulle. Algeria is already independent, there is a clear succession system in place until such time as the new elections can be held but the October 1962 referendum on direct election of the President hasn't been passed, indeed it hasn't been mooted. This is important because every party, even the UNR, opposed it to some levels, some due to the power it gave to the President, some due to the fact that they thought this was a matter for Congress to resolve, not the population. This was THE moment when de Gaulle established his bonapartist model of governance, when he proved that the president could defy every party including his own and go directly to the French people. How bad did he break the parliamentary model, nearly a century old by that time? The Assemblée passed its one and only motion of censure of a government (Pompidou's), de Gaulle dissolved the Assembly and, in stark contrast to when Mac-Mahon did that in 1877 and lost, de Gaulle won a renewed majority, one entirely under his thumb and went ahead with the referendum.
If he dies just before that, and the attempted assassination certainly strengthened his resolve in going ahead and trusting in his magic touch with the French people, then the Fifth Republic is a lot more parliamentary. The Conseil Constitutionnel is still nine years away from asserting its importance. Nobody has the legacy of de Gaulle to make an imperial presidency stick and nobody really wants it.
Mitterand makes a bold move when Germany unifies idk
I'm just going to say no to that one. It's true Mitterrand wrote
Le Coup d'Etat permanent to protest against de Gaulle's seizure of power and his consolidation of it, and a lot of the arguments have actual legs. But Mitterrand's attitude in l'attentat de l'Observatoire (where there has serious signs he organised a failed hit against himself to profit from sympathy), his grasp for power in May 1968 when nobody knew where de Gaulle had gone off to point against this. What's more, he made two very serious attempts at the Presidency in 1965 and 1974, putting de Gaulle in ballotage in the first one and coming within a breath of defeating Giscard in the second. Even if he knew he was dying of cancer by 1981, he still wanted it. In fact, one of the major rebukes the PS had for him in 1988 is that he campaigned exactly as the sort of man above parties that de Gaulle had cast himself as. In fact, it cost them dearly in 1993.
Based solely on the strength, length and depth of my hatred of the man and everything he stands for, I flatly refuse to even engage in speculation on that one. However, I'm ready to discuss another candidate for pushing it (gee, Angelo, is there anyone on the left you don't hate? yes, they lose in embarrassing fashion). The up-and-coming man in the early 00's wasn't Mélenchon, who was a undersecretary to Technical Education, and only started to make his name in the 2005 referendum and his brilliant handling of his leaving of the PS afterwards. It was Arnaud Montebourg. The man had been in the news under Jospin for a petition against Chirac's presidential immunity and wanted him tried in the Haute Cour. He was a very big agitator on the left and he was the one to consistently agitate for the Sixth Republic, more or less on the same ground than Mélenchon. He was very much the white knight of politics, whose crusade was getting the crooks out.
Now, let's suppose the Méry cassette tape is a lot more incriminating against Chirac (or that Chirac doesn't channel Rimbaud, call the accusations "abracadabrantesques" and manages to deflect media interest for weeks because look at the shiny culture!), or that hard proof emerges against Balladur in the 1995 campaign funding scandal, or that Roland Dumas and Bernard Tapie talk during their trials (okay, that might be close to ASB), Chirac might actually find it impossible to run in 2002. He might, in any case, never go for the referendum shortening the presidential term to five years, even though Jospin was also in favour. The butterflies could be interesting. Montebourg certainly could have been a possible PM for Jospin in a 2002 to 2007 or 2009 term, and he might have aggressively pushed for a referendum on the Sixth Republic.
Macron literally decides to write a new constitution in his second term (technically FH but this actually might be the most plausible one)
This one I'm putting under 'doubtful'. A few things I believe about Macron is that he's both very clever and very soon bored of something and wanting to move to another challenge. He's probably the best President to understand the incarnation you need to put into the function since Mitterrand, something Chirac got to only a lesser degree, and that neither Sarkozy nor Hollande managed to get close to except on a few of the more solemn days of their terms: if only by his mastering of the malaise and riding it to the Elysée . And for all the derision he got for a Jovian concept of the presidency, I think he truly believes that, by being the only guy brilliant enough to identify the problems and the solutions, he's perfectly suited to solve everything, and for that, you need a Gaullian presidency. So, unless he thinks we're all wasting his time and he wants to send down the ship out of spite (and even I don't think that badly of him), it's not really what he would do.
One POD I'm going to suggest, although I'm far from knowledgeable on its specifics, is the survival of the Programme Commun (1972) which involved a lot of institutional changes, one of the most notable of which was the abolition of the dreaded Article 16 in the Constitution. It broke down some time after the 1977 election, and the fact that the PS did better in the législatives of 1978 than the PC for the first time was a sign to Mitterrand that he could now win the presidency without an explicit alliance to the PC. So, either an earlier and more severe cancer for Mitterrand, or Chevènement' CERES throwing its lot in the 1979 Metz Congress (by this point,
@OwenM probably knows the PS's bylaws and its inner politics better than I do) for Rocard and Mauroy: this one might be far-fetched, because Chevènement and Rocard don't mesh really well, but it could be a temporary alliance to undo Mitterrand and the chance for Chevènement to extract a significant pound of flesh from Rocard that he would not get from someone closer to him in the left of the party.
In this case, a differently-led PS, which might or might not win in 1981, still on a Programme Commun basis, might actually enact the institutional reforms it promised. The changes could either led strictly to a complete overhaul of the Constitution, or be so in depth that functionally the Republic is no longer really the Fifth even though it bears the name. Even if they don't win, that means VGE is a two-termer and would bring a different sensitivity from the center-right to reform the Republic, probably to bring it as close as he can achieve to European federalism.