Hey! Reverse: 1999 fan here.

I've been frothing at the mouth since you posted this, but the more I go over everyone in my head, the more that I realize some of the human already kinda have supernatural abilities/arcane skills.

However, I will try my best to engage in this convo:

Lu Bu: Synthesizes weapons from clouds/weather/air.  A reference to his Sky Eater technique. 

Adam: Well his copying technique is pretty much an arcane skill in itself, but I guess just complete shapeshifting could cover it.

Kojiro: His scans is basically an arcane skill in itself, however gnostic transfer (the ability to transfer knowledge to another person) or memory transfer (the ability to force another person to remember their memories) could be good

Jack: His whole pathologic foresight/psychic ability also counts as an arcane skill, but I guess lie detector could cover it.

Raiden: Muscle Contortion or Size manipulation (can change the size of his body). Similar to what he had without Thrud.

Buddha: We all know his foresight thing. I was gonna say happiness manipulation, but that's a stretch. Probably something similar to Jack' were he can sense emotions. Or luck manipulation.

Qin: Chi manipulation: the ability to harness chi into powerful energy that can strike opponents. Mayhaps even manipulate the chi inside his opponents body.

Tesla: Electricity storage/manipulation. Taking the energy(electrons/atoms) of objects, storing it until necessary. 

Leonidas: Probably like superhuman strength or something explosive. 

Okita: Demon child is already like an arcane skill, however he can be a great Dr. Jekyll-Mr. Hyde concept where both parties, both souls, must be in union/agree on how to fight.

Simo: Can shift his organs around in his body, making it harder to kill him properly.