>>51480657 (me)(I guess Pikachu and a few other Pokemon could probably fuse, it'd be a selection of maybe 20-ish Pokemon across the whole game.)
Anyway, it'd probably be a regional variant of a Ditto, where instead of transforming into things on its own, it would have a reaction with some Pokemon and give some aspects of another Pokemon to it, as well as adding the regional Ditto's pitiful stats (adjustable via IVs and EVs, as always).
Gameplay-wise, you'd be sacrificing one of your six slots for this Ditto, and the fusion would likely be your Pokemon plus a Pokemon that would go well with it, like a Lopunny with Gardevoir to turn it into a blazing fast psychic type, or an Eevee that gains kind of a rainbow motif (for the rest of the eeveelutions) and gets a bunch of raw stat increases.
Maybe a fused Pokemon would also get a fifth move, or would convert moves like Z-moves and the max forms did.
Also, if the Pokemon faints, both it and the Ditto faint, but you can still revive them using your preferred/allowed methods, and if you switch the fused Pokemon out, they split.
