I feel like there is some potential with genjutsu as sort of a support / diversion role.
Maybe a skill could be something like "If an enemy is hit with it, their screen goes pitch black (can only see their player) for like 3 seconds" or something like that.
Another skill could possibly be "If an enemy is hit with it, you will disappear on their screen for 2 seconds, but when you reappear there will be a clone of you that doesn't do damage, but needs to be hit to be destroyed" which can possibly buy you a few seconds.
This one is getting a little risky -- but maybe something along the lines of "anyone in range of the attack has their skillbar randomized/jumbled/switched around for 3 seconds".
Another one could be "The enemies HP/MP bars are shown as 0 for 5 seconds or something similar so they won't know how low/high they are.
I'm just spitballing here, but I think it could be really cool and a nice niche/unique mastery that could be super useful for team battles, or maybe escaping an attack.
The problem is, I'm not sure if this is to OP, or too UP. Also, I'm not quite sure what to do about damage dealing attacks. Obviously, you'll need them for grinding/1v1s, but I can't think of too many.
Maybe something along the lines of - "Creates a few invisible pits and any enemy who is standing on them will take constant damage".
Feel free to add on any suggestions, or tell me how much you hate it
Also, I'm not sure how you could really use most of these skills for grinding since you'll be fighting NPCs