Play as Boss
Beat the game using all 20 characters. Then at the character selection screen, press Down at the leftmost or Up at the rightmost fighter.
Alternate costumes and more options
Beat story mode on the easy difficulty setting. Then at the character selection screen, highlight a fighter and press L2 to choose a new costume. Additionally, the number of continues and rounds may now be adjusted in the sound test screen.
Slow motion replay
Hold Select before the replay begins.