What dead video game character would you bring back to life? This can be either in the literal sense (ie. Cole McGrath) or in the figurative sense as in a dead franchise (KILLER 7 SEQUEL C'MON!).
Please God don't say Aeris/Aerith (whatever floats your boat).
My own opinions follow: I would bring back Joker from the batman series. The new batman just doesn't interest me because it's lacking Joker. Is that stupid? yes probably but for me I really enjoy playing games for the stories, and the Batman/Joker dichotomy always resonates with me. Batman vs. Copperhead? I don't know.
They could easily bring him back I feel using the ole, "It was all a plan to get Joker free!" In the end of the game you see Batman himself, carrying Joker out of Arkham city so that seems totally like a thing joker would do - have his arch nemesis walk him out of his prison. You'd probably have to do some light retcon in order to make it work but I don't think it'd be anything too terrible.