I'm kinda working on that, myself, and I've got a few ideas in my head, as far as the lore goes, HOWEVER, the whole 'closed door' thing is absent in this... shadow pokemon are created either via direct contact with, or being the offspring of another who's had contact with an old and lost super-soldier serum from the war in the timeline. They tend to be more aggressive than most, but aren't always flat-out evil.

Evolution works just based on just about what level you WOULD be if we were keeping track, it's a much looser system, so after a few dungeons, or something, you could evolve a cyndaquil into a quilava.