Play As Ultimate Rugal:
To play as this character beat the game on default difficulty in ratio mode. after the credits you will see a messege comfirming you did it.

Play as Shin Akuma:
Beat the game in either ration mode or 3 on 3 mode. If your groove points are high enough, you may get to face Shin Akuma. Beat him (If you can) and he will become a playable character.

SubBoss and Boss Fights:
Before the final battle, you may get interrupted to face one of two subbosses, either M. Bison or Geese Howard. This will only occur if you have over 700(?) groove points. After the final round you will fight one of four bosses if your groove points are high enough. If you have over 800(?) groove points you will at least face either Akuma or Rugal. If you have over 1000(?) points you may get the opportunty to face either Shin Akuma or Ultra Rugal.

Special Starts:
When two characters start off a round against each other, they may perform a special pose or action. I have discovered 14 different ones so far.

Ryu VS Ken           Rock VS Geese
Bison VS Iori        Hibiki VS Rolento
Chun-Li VS Yamazaki  Ryo VS Yuri
Terry VS Geese       Kyo VS Iori
Ryu VS Kyo           Terry VS Ken
Bison VS Geese       Ryu VS Sagat
Sakura VS Yuri       Raiden VS Zangief

Thanks to Revolution readers Mokojean Strider Hiryu and Ethan Neufeld!