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Unless the games go through a HUGE overhaul, I think it'd be pretty hard to introduce new types at this point desu
Fairy was vague enough that a lot of things could be retconned into it without affecting the game too much, but things like Sound, Light, Cosmic, etc. would require a lot more than just "add the new type here and there and retcon a few Normal-type mons and moves"
I could see a "Typeless" type being added at some point, though. But it would be super rare and restricted to things like Ditto, Castform, Arceus and Type: Null
