So, I was thinking abut a side evolution method for rearranging different stat distributions for evolutions. So, here is an idea, Rebound Evolution.
Basically, this would be a "hidden" evolution method achievable after completing and refusing the conventional evolution, by pressing B button. For example, let's say you use Thunder Stone on a Pikachu. You stop it by pressing B button. The Pokémon resets and now you can evolve it using a different method, conventional or otherwise.
Some sort of key or hold item might be needed to trigger these evolutions, but basically, the idea would be that you would go back from evolution and "correct" it to get a rare variant. This variant will likely not be different BST, but it would allow you to get one with different, generally more harshly min-maxed or differently typed variant of the evolution that would serve as the dex filler.