Okay listen, who thinks that Cole Macgrath, the man who was able to save the world and kill himself in the process, should come back? Personally, I think he shouldn't. It wouldn't make any sense. Now don't get me wrong, Cole was a badass, but he had to die for a reason. There was no way out. He either became the monster he tried to stop, or wipe the entire Conduit race.
Now off that note, I think that the story of the next installment of InFamous should have a new hero (Delsin is cool and all, but you can't really build much off of him unless you pick the GOOD ENDING.) The new character should have Zeke as his/her first partner in the game. Your character and Zeke go to a city that is NOT REAL. It would give the designers flexibility and could make it look amazing. The villain could possibly be looking for ways to have the ability to take control of other Conduits or some random crap like that, but at one point in the story you should be able to go to Cole's grave/tomb, or you could go back to the monument at New Marais to get Cole's Amp. That would be awesome. Imagine the character pulling out the Amp from the monument and using it to channel his/her powers? That's as far as I could go on, but add on what you would like.
Also as a side note, please bring back the lightning powers. They were amazing.