Here are some of my ideas:
Nightwolf - Scalp Himself (this would be classic)
Baraka - Place both of his fists on his chest then extend his blades, having them pop out his back.
Scorpion - Do a true hari kari and eviscerate himself with the ninja sword, have the insides flop out and everything.
Kira - Black dragon style pull her own heart out.
Bo Rai Cho - fix himself up a molotov cocktail then light himself aflame with it.
Kabal - if he has his respirator, remove it and having him dying of respitory failure.
Kenshi - Levitate his own sword up above his head, then bring it down through his skull.
well i got a few started, anyone else have ideas?