That would be awesome. I like the idea of location specific evolution. For example, the Ruins of Alph. I always thought there would be more to that place than what was in the games. Than just a specific kind of pokemon. Explain it as an ancient energy that allows you to get a now extinct evolution of the various Starters. Maybe Mewtwo's cave on Kanto
Additionally, I'll look for some pictures of potential evolutionary Grandfathers for the Silver starters. I would love to evolve Feraligatr another step
Here are some Gen 2 starter grandfathers: