From what I can tell, the way Martial Artists in DF function is that they maintain a special diet and do special daily exercises, and as a result have access to a variety of superhuman abilities. They also have access to the more advanced unarmed combat skills (Karate and Judo).
It occurs to me that an in-genre approach may be to have Chi be "scientific" in nature - after all, Karate (and Boxing) and Judo are often considered more scientific approaches to combat (as opposed to the more instinctive Brawling and Wrestling). In this case, careful regulation of diet and exercise grants one a state of more balanced humors than a normal person can achieve, unlocking true human potential, and allowing them - with appropriate practice - to do the sorts of things legendary heroes could achieve, like quelling a large number of foes with a single shout (Kiai), breaking a massive stone with a single strike (Shattering Blow), leaping over canyons (Flying Leap), running over thin ice or leaving no trail over soft mud (Light Walk), etc. Just give Chi a different name and fluff it up as being a consequence of supremely balanced humors, and there you go. As for the name, apparently "humor" is a translation of the Greek word χυμός, which is pronounced something like
chi-mos... so in theory, you could actually
still call it Chi, with the understanding this is short for χυμός (which, according to Wikipedia, literally means juice or sap, and is metaphorically used to mean flavor).
EDIT: I'm not sure what you'd actually call the class, however. Also, some of the more
anime-inspired options that show up in later books/articles - like being able to throw chi blasts -
may not be appropriate, but I think the mechanical core of the class as it was presented in DF1 would be maintained.