[blockquote]Spoilers[/blockquote]
I've got a good idea for an Infamous 3. The major problem with a 3rd installment story wise is that the endings are so different, in one Cole dies and the world presumably begins to recover from the disasters that have happened since the story began, in the other everyone who is not a conduit is presumably destroyed by Cole's conduit army.
So, how about a game where these two different realities colide? Imagine that both good and evil endings of both games are canon but that they simply take place in different universes. Something happens (don't ask me what cause I'm not sure) to bring these two universes together, or to transfer one character from one universe to the other. Good Cole is somehow brought back to life in order to fight his evil alternate. This way you can still play the game through twice as either good or evil Cole, it allows characters such as Zeke or Kuo to return, it retains the basic premise of the first two games (i.e being able to play as a hero or a villain) AND it would give Cole a true nemesis for the final boss: someone he shares all of his abilities, but slightly tweaked according to either good or evil alignment. I'm not saying it's perfect, but not a bad starting point for an idea is it?