I think if it is implemented it would be done by regional variants, at least at first, since it would give some more in-universe justification for why these split evolutions are showing up and why such a thing would occur. One good demonstration of this concept would be Bulbasaur; you can just switch the growth on its back. I can see a Grass/Fairy Bulbasaur that uses a mushroom (you know, a literal toadstool) and becomes a "Fungasaur" in its final form. If you want to do a regional variant of a starter that's not a starter type (say, you wanna experiment and do another type triangle) for most starters I think it would be necessary. I don't think it's out of left field for Gamefreak; you could breathe a lot of new life into old starter Pokemon. And I mean, isn't that half the point of regional variants?