Notice that the select screen seems to have room for two additional characters.
Oni Akuma and Evil Ryu have become playable in SF4: Arcade Edition. They both look more different from their standard versions than they have in past games. Oni has a Gouken-style chargeable fireball, though it only goes straight. He also has a mid-air dash that is an attack and can go forward, backward, or down. He has a dash-punch that can go through an opponent and hit from behind. He has a quick jumping attack that hits overhead as an EX. He also has his standard 3-hit Shoryuken and Tatsu.
Evil Ryu has a super-fast dash and a new axe kick that hits overhead as an EX. He has Ryu's F+MK hop kick from the Alpha games.
Both Evil Ryu and Oni have a Raging Demon as their Super and a fireball as their Ultra 1. Oni is able to perform his Ultra 1 (and maybe even his Super, ala Shining Gou Shock) in the air. Oni's Ultra 2 is a close-hitting attack that does a ton of damage. Evil Ryu's Ultra 2 is a Messatsu Gou Shoryu, which is Akuma's old uppercut Super from Alpha and Third Strike.
Oni has a fairly normal amount of health, while Evil Ryu's is slightly low. Evil Ryu, however, is faster and dishes out more damage. Will these characters be top tier? Well, that remains to be seen. Initial impressions are that they're not as obviously overpowered as Yun and Yang. I think Oni's power will be in his air mobility and his ability to set up cross-ups with his rush punch. Evil Ryu will be all about using his speed to get in position to land damaging combos. He can supposedly link c.MPx3 as well as EX fireball, FADC, Ultra 2. He might even be able to set up sneaky resets with his overhead axe kick. It will be interesting to see how these characters develop.
Shoryuken - Video and Images of Oni Akuma and Evil Ryu in Super Street Fighter 4 Arcade Edition
iPlaywinner - Exclusive Oni Akuma and Evil Ryu Media, Movesets, and Images