You get to make a new Pokémon type, but only if you can give it to at least one old Pokémon and make three reasonable regional forms.

What pick?

I’ll go first

Nuclear
Retype:
Kantonian Muk (Poison > Poison/Nuclear)
Regionals:
>Tropius (Grass/Nuclear, based on bananas being radioactive) which evolves into Isotropius
>Gigalith (Rock/Nuclear, based on granite being radioactive)
>Sharpedo (Nuclear/Water, now a warhead instead of a torpedo).