I hope they stay away from anything fantastical for the next one. Keep it grounded in reality, man.
Well, someone like Mr. Freeze can actually be done in a realistic manner -either cryosuit or just a guy with a chemical freezing agent- but villains such as Clayface (the shapeshifter), Killer Croc and Killer Moth.
More likely they would go for someone like the Riddler, but take away the costume but leave the personality.
Also really hope that they don't do a big team up or multiple villian angle. Sick of films that do that (looking at you Spiderman 3!), unless they do it as lesser villians such as Deadshot (a suicidical version of Bullseye) and other thugs under a ring leader such as The Pegnuin (in his more modern comic version) or Hush (though they should build up to him later if they want to go down that road).
Catwoman can go in if they keep her to the traditional human thief and sometimes murderer (depending on which depiction and continuity she's from) but she wouldn't go well for a whole film.
Same with Ventriloquist and Scarface.
though, in all honesty, I'd prefer to see them stick with more Batman vs the criminal underworld and staying more away from the Super Villains and the like.
Keep the franchise more in reality but still have stuff that allows us our suspension of disbelief.
yeah, these kind of films really do bring out the ubernerd in me.
But after movie studios finally getting superhero movies more akin to right and treating them like films, it does get your hopes up that they will keep up the trend.
Last thing we want is another Batman Forever/Batman & Robin or, Satan Forbid, Nick Fury as played by Hasslehoff