Its the perfect typing for ultra beasts and would've actually made them more than just glorified legendaries.
>how would it work?
I was thinking it would have a unique gimmick. Each pokemon who has the ??? type would have unique resistances and weaknesses. For example, Buzzwole would be ??? type and be weak to psychic, fighting, and electric while Nihilego would also have the ??? type but be immune to psychic. As for ??? type moves, they would work like hidden power. The type of the move is determined by the pokemon using it. For example, Xurkitree's ??? type attacks will always deal electric type damage while Guzzlord's ??? type attacks will always deal dark type damage.