The game may not have come out yet but consider: scarlet and violet tera power au where when the trainer uses tera powers they also gain Cool Crystal Hat. Use them frequently, parts of ur body/hair will turn to crystal while ur using them. Use em more, larger and larger sections of your body turn to crystal. Misuse them, when ur pokemon de-crystals it takes you a bit longer to turn back. Keep at it and eventually you just stay a crystal.
Also:
bigger hats for more expirenced trainers !
It's not much of an au yet but i think it's a fun idea and i wanna play with it, and really encourage others to do the same!! I'm a big fan of lowkey body horror au stuff, and none of the pokemon gimmicks have suited this sort of thing better than tera powers imo! if anyone else wants to make stuff for this kind of au PlEASE do and PLEASE show me (and if someone else has alrady dont this fuck yeah i wanna see also same braincell)