In the original reveal trailer there was an ice beam attacking Flash. Odds are this is Captain Cold since he is a well-known ice based Flash villain. That said, I can't help but wonder what kind of fan reaction there would be though if this turns out to be Mr. Freeze, Sub Zero, or someone else. Personally I like Mr. Freeze the best and would be happy to see him.
For some other ice based contenders see Robot Chicken DC Comics special (3:20 - 5:10): http://www.youtube.com/watch?v=BgnuaK4bcY0