Deadshot was in Gotham Knight already, I would be surprised if they brought him back.

I would love to see Catwoman (NOT played by Angelina Jolie, wtf is wrong with people). I agree that the Riddler seems like a low-rate Joker, but done right he could be fantastic, and he's probably the most likely choice. A great chance to finally examine those detective skills of Batman.

My personal (read: fanboy) choice would be to see the Ventriloquist/Scarface, but because they're pretty minor, I expect if ANYTHING they'll be a cameo, like Zsasz in Batman Begins.

I'm not familiar with Black Mask, but he sounds cool.