There are a lot of rumour floating around. Superman, Ninja Turtles, G.I. Joe, but the most pervasive rumour right now is a Batman prequel set in the Silver Age. Not my cup of tea but given what we've already seen from Rocksteady I'd love to see what they can make of it.
What we know so far is that there are in fact two Batman Arkham games in development: Batman Arkham Origins for the major platforms and Batman Arkham Origins Blackgate for PS Vita, which is still secondary but supposedly more substantial than the last portable Batman Arkham game and it's gonna be Metroidvania style. It might get ported to other portables but no official word on that yet.