In a recent interview by famitsu, Ono san hints that another character who was only available in Street Fighter Zero 3 might join the cast.
If you think about the sfz3 roster there are only 2 characters who were exclusive to the game: Cody, Mika and Karin.
Cody is definetly out of the question as Ono san stated before that no final fight character will be in SF4, leaving either Karin or R.Mika to join the cast.
Some people have already predicted that it is probably Karin since Sakura is in (Karin is Sakura's rival in the cannon Street Fighter storyline)
Only time will tell