In gameplay terms ki would have to be narrowed down to an exact function.
Here are several ideas I had off the top of my head:
* ki shield - curves paths of incoming bullets
* ki-flect - opponent's own melee attack knocks him down
* ki fart - you travel much faster between towns
ok I'm running low on sugar, need to eat something to replenish my ki
Don't some martial arts like Judo use the oponent's weight against him to knock him down? This would be interesting in a setting with heavy armor that can't be penetrated by punches.
Looks like we've arrived back to the weird melee guy taking on power armor guys scenario... only this time with the power of LEVERAGE!
So you get close to him pretending to be his friend and then BAM he's on the ground... but if he doesn't let you get close, you're perforated by his gun, because good luck trying to do joint locks or anything while wearing a power armor yourself...
ok i need food