Hidden Bosses:
You can play as hidden bosses once they have been beaten. The conditions of the Bosses to appear are to meet two out of the following three conditions. If you play with a Capcom character you will get God Rugal, and if you play with a SNK character you will get Shin Gouki.

Get over 1500 GPS by the end of the game.
Beat the Middle Boss (Geese or Bison/Vega).
Achieve at least one Finest KO.
Custom Grooves:
The Custom Groove option appears once a normal Boss has been beaten. Normal Bosses appear by meeting two of the four following conditions.
Get over 1000 GPS by the end of the game.
Achieve four or more special KOs.
Middle Boss has appeared.
Achieve two or more LV3 super combo KOs.
Random Groove:
Press Start at the Groove select screen.
Boss challenge mode:
Boss challenge mode becomes available under these two conditions. Both hidden Bosses (God Rugal, Shin Gouki) must already be beaten and unlocked. Hold Triangle + Square when choosing arcade mode.

One on three tag team mode:
Highlighter any fighter at the character select screen and press a button. Allow time to run out and the game will start in one on three tag team mode. Do not choose one on one battle mode.

Extra options:
Finish Boss challenge mode with no continues. The "Extra Option" becomes available in the game.

Extra color edit mode options:
Press Start when editing the colors for your character to display more options.

Infinite custom groove:
Beat all characters straight in survival mode. The custom groove option settings will become infinite.

Middle Boss conditions:
The middle Boss appears when these conditions are all met:

No time over has occurred
The CPU has not made over six "first attacks".
The CPU has not made three or more Special KOs.
Victory quotes:
Hold Start + FP after winning a stage to make your character say a specific quote. Hold Start + FK to make the character talk with a team member.