Another thing rattlin' 'round in ol' noggin about the Wednesday au is Ibara being the headmaster and obviously an antagonist, but idk what he'd be race/species wise.
What are your thoughts, it can be types from Wednesday or just of supernatural/creatures in general.
Ibara being the headmaster/antagonist is perfect! Maybe he's trying to mind control the students into becoming his slaves. Or trying to take their life essence and turn it into power. Either way, that would make Ryusui, a normie he can't take anything from, a thorn in his side. Another thorn is Senku, who can't be mind controlled and is able to take apart any apparatus he uses for his goals.
(I like the plot of Ibara's plan focusing on outcasts since that would make Ryusui more significant than being just... there, you know? But that's just my two cents.)
As for the species, there are two possibilities:
If we go by Wednesday world building, I'd make him a shape shifter, who once upon a time imprisoned a Gorgon's (Soyuz's father's) power in some trinket (maybe a ring) and uses that stolen power to turn into stone the people in his way. Kinda fits his whole arc in canon. Plus, it would give him another reason to hate Senku since his handy little trinket can't do squat against an actual Gorgon.
Plus, having him be a shape shifter just like Gen could give you a reason to make a sorta season 2 or an arc for after his defeat where Gen agonises over being part shape shifter and if that makes him as despicable as Ibara. To the point where he clams up, retreats into himself and straight up won't use his shape shifting anymore. Until the climax where his friends again put their noses where they don't belong, get into danger and he HAS TO use shape shifting to rescue them.
Or if you decide to step outside the established species, the sky's the limit. You can even make him some kind of cryptid that was never seen before. With everyone else panicking about how they're doomed because how tf can they defeat something they know jack shit about?!
Meanwhile, Senku is excited. Something entirely new means he can unleash his full gremlin nature and experiment to his heart's content with absolutely no holdbacks! He'll make sure to squeeze it out for all it's worth! With Ryusui encouraging him since for him as a normie all of this is new terrain, so what's one more monster? Chrome is excited because an excited Senku means BAD science! Gen and Ukyo are feeling at how tf they ended up with these guys of all people, and bonded to the two biggest instigators.
