I was thinking that maybe pokemon can evolve if it has a certain sickness like Pokerus, even though it'd be extremely rare to evolve a pokemon from that, it can be quite interesting, imo. And another thing, how do you guys feel about evolution that is more deeply involved with DNA? Or perhaps more odd and hidden evolution methods? For example... there was that time where Nincada can technically evolve into both a Ninjask, and a Shedinja, so long as you've had a pokeball and a empty slot in your party. Could there be one pokemon that you have to have at least one other pokemon to evolve the other pokemon with?
Or with letters:
You need Pokemon "A" and "B" to evolve Pokemon "A" to Pokemon "C".