Kaioken would be a nice attack, and it would even be better to add when they install the new melee system.
If it was up to me, i would make Kaioken..
You know how you charge beams? well each beam you charge has little bars where depending how far u charge, the stronger yout beam attack will be. THis is how i would do it for Kaioken, 4 bars = Kaioken, x2, x3, and ,4.
Also to make it not so overwhelming to the rest of the fighters you should add downsides to it. Lets say normal kaioken upon usage, you will gain speed and pl for 5 sec, not 45 sec like FlexCon up there mentioned lol. Also when Kaiken is on, you cant use power up(turbo) since this will overide turbo for the next 5 sec.
Kaioken will add 500,000 to your pl and 25 to speed/lose 20% hp
x2 will add 1mill, and 50 to speed/lose 40% hp
x3 will add 1.5mill, and 75 to speed/lose 60% hp
x4 will add 2mill, and 100 to speed/lose 80% hp
Also since it takes away % hp, the fighter who is using this technique will have to watch it due to the fact of less life = quicker death if caught. If you try Kaioken for the 2nd time without the required % of life then you will die.
This is how i would make it so for Kaioken.