Retro Pokémon are a great addition to the site itself as it is.
They are super rare and they look very cool. Being able to evolve
the Retro Pokémon would make them look even cooler and it is not
really game-breaking either. All you have to do is find the sprites
for the Retro Pokémon and make them evolve with the same
requirements as with regular Pokémon, i.e. Retro Bulbasaur evolves
into Retro Ivysaur at Level 16, same as with regular Bulbasaur. An
example of a Retro Sprite for example Ivysaur would maybe be
this:
.