Tatoruzu I think a TMNT "cinematic universe" would be the death kin of me ever funding/ supporting another picture of these characters.
Eh, I mean, you have to think in terms of what the general audience perceives these characters as. Doing a slasher-esque movie with The Rat King would have to be EXTREMELY noteworthy/ innovative to get their butts in seats; cinematography, editing, and an actor to go alongside that. Do it like the old Hollywood way where the actor(s) sold you on the movie, and not the other way around like nowadays.
But, like I said, I prefer if he was more of a secondary antagonist. I'm not completely sold on the idea that The Rat King could work as the only antagonist, but hey, I might be wrong.