I'm guessing it will probably be a special alt costume like the Challenge Tower in MK. If it were a secret character, I'm sure the competetive players would be upset because unlockable characters are just a hassle for tournies. An alternate costume is something that isn't too big to make, and is still something cool that you can show off online to prove you accomplished something (unlike a secret mode or something, which you can't really show off online)
Maybe a sexy costume for Killer Frost where she is completely naked except for little snowflakes covering her naughty parts. I doubt DC would allow them to do a slutty outfit for Wonder Woman, or any other of the really BIG name females.