I have a couple of ideas, from the fairly mundane 'enhanced physicality' (based in adrenal enhancement) to full on Nightmare Enducing Demon-form (Hormonal/pheromone based). If I had to condense them down, I've got:

1) John Wick on Speed, Steroids and Endorphines (A physically enhanced gunfighter)
2) Kaminari and Raiden's angsty lovechild (Lightning based powers)
3) Human Darkrai with a liberal dash of Scarecrow (a combination of fear and transformative based powers)
4) Nanomachines, Son (seamless Biological integration and interaction with technology)
5) Unlimited Blade Works (blade-based Psionics)
6) Angery Dragon-boi (draconic mutation)

So pick which sounds the best and I'll go with that, I guess.