>>40969320As a Mechanic B/W Kyurem and Sun/Moon Necrozma already did it perfectly. Legendary Only feature, Give up a mon for Form Change, can remove the Boosting Mon to revert back to normal form.
Though honestly i also liked how SM handled Zygarde, tons of "Pieces" scattered across the game world, get a stronger form on specific thresholds.
I mean, technically there are quite a few "Fusion" mons already, even back in Gen1. "Abomination" ones too considering Mewtwo had shit spliced in IIRC.
