Well they're is the PAX(Penny Arcade Expo.) coming up on Aug. 29th.

http://www.pennyarcadeexpo.com/

I don't know if Midway plans on attending, but it is the next big games Convention. That may be the next time we get characters revealed. Don't take that as a for sure, as I am just taking a guess. I would say that if not then, than maybe within the next few weeks. They may save one or two as secrets though.

hopefully soon man!!

I'm almost positive MK vs DCU WILL be at PAX. I've seen it listed in previews for the event. Edit: Here it is: http://multiplayerblog.mtv.com/2008/08/21/direct-from-sf-pax/