Was idly thinking earlier about whether it's possible to have a team that covers all 18 types at once, or at least as near to 18 as it's possible to get* - because while you can obviously only have a maximum of 12 base types on a team, surely there are several Pokemon with interchangeable forms which swap types, artificially adding to your number.
But actually no, there's... very few Pokemon which change type with form changes? I could have sworn there were more. To clarify, there are plenty of Pokemon which acquire or lose a type (i.e. go from single-typed to dual-typed or vice versa) like Darmanitan and Shaymin, and plenty who simply swap around either their primary or secondary type, like Rotom and Oricorio, but I could have sworn there were a lot more alternate forms that change their type entirely.
But no. Looking at the list of Pokemon with alternate forms, the only ones who change type outright are...
- Castform - goes from pure Normal to pure Fire, Water, or Ice
- Arceus - can be any of the 18 types, but only one at once
- Silvally - as above
This strikes me as such a massive gaping hole from a creative standpoint: after nine generations I'm really surprised that the potential for this mechanic still remains (almost entirely) untapped. Of course there are several Pokemon with non-interchangeable forms who are completely different types: Vulpix, Sandshrew, Darumaka, Tauros, et al. But a Pokemon which was, for instance, pure Water-type most of the time but could become pure Fire-type some of the time is a novel concept and a really cool idea, and it's funny that this hasn't been done.
Is it that it'd potentially be too powerful? I don't really see that. If we were to use the Water-becomes-Fire model as a concept, presumably it wouldn't actually learn that many Fire moves anyway since the base form would be Water (that's assuming the change were an entirely temporary in-battle one - it could just as easily be a prompted at-will out-of-battle change triggered with an item or something). Sure, a Fire-type which learns several Water moves would have an easy answer to any Rock- or Ground-types that tried to counter it, but it's not as if there aren't existing Fire-types that learn Grass, Steel, Water, and Fighting moves. So I don't think that can be it.
But yeah. Kind of feel like they missed a trick here.
*Terastallization aside, of course