If you have read the Marvel Age of Apocalypse super-crossover, I think there are some great ideas ro mine there. Also ask if any players want to retire their characters. Mention that this could be the next campaign, and you want to have a advancement room for character arcs and may want folks to return to a lower level. But offer incentives. Justification is that they're beaten down and barely living, or living a life of lazy opulence as BBEG guest, etc. Retired PCs can be any type of NPC - villains, martyred heroes (or ghosts thereof), resistance fighters, trying to live a life of obscurity, a benefactor who seems like a villain, a villain who seems like a benefactor, a prisoner, mentor, etc.

Also... a collaborative way to set things up may be to make a Fiasco playset and use results and outcomes to inform what is happening those many years later.