In a post by Natsuki Shiozawa on the official SFIV devblog a few hints have been dropped that a new character will become unlocked in Street Fighter IV on September 26th.
Shiozawa drops hints that this character has never been playable but we have seen a glimpse of him before. Since Akuma has already been unlocked and spotted in arcades around the world Shiozawa suggests we think about who would logically be the most likely character to show up next....
This guy. Gouken, Ryu and Ken's master.
The word is that he won't be playable yet so he is most likey going to show up Akuma-style when the player is fighting a little too well in order to show you pansy asses how it's done :)
Personally I'm not good enough at SFIV to come across Akuma yet so I am guessing many a $2 coin may have to be thrown down if I want to come across this fella any time soon.
Found on the Street Fighter Blog