Since it does seem as though your past games revolve around the Marvel Universe, here's my favorite campaign idea for pre-existing gameworlds (I'll stick with Marvel for this example)...
Last summer, during one of those seemingly-annual bruhahas (sp?), every single participant suddenly disappeared. Heroes, villians, sidekicks - - all gone!
Now, a new threat has arisen - - say, a Skrull invasion. But the Avengers, FF, et al are gone. Who will save us?
Enter the Watcher (who you know will interfere all the while saying he's forbidden to do so). He summons those who are best suited to deal with the current crisis (i.e. the PCs), and equips them with immediately identifiable items/powers...
-Thor's hammer (1 PC *must* be worthy)
-Cap's shield
-a suit of Iron Man armor
-access to the Enigma Force (aka Captain Universe)
-etc
So, the campaign is two-fold - - stop the invasion, and figure out what happened to the heroes (and why is all their stuff still here?).
Want to switch to DC? Replace accordingly...
-GL Power Ring
-Dr Fate's helmet
-Hawkman/Girl's wings
-Batman's utility belt
-etc etc
The upside is PCs get to play favorite characters, but with their own spin (i.e. Thor as a rapper, a fashion model as Iron "Man", etc etc). Downside, of course, is the powers are pre-set - - but a good player should easily be able to overcome that. After all, the heart of a character is the *character*, not the power-set.
Anyway, that's my thoughts.
-Yogzilla