I think that the gimmick should some sort of genetic mutation, where the pokemon gets very strong for the rest of the battle, but slowly takes damage over time. Also, certain pokemon can mutate into a different pokemon, similar to a mega evolution. Please leave your comments about it and if it is a good Idea.
SLIGHT UPDATE Together we have come up with some new ideas. The name of the gimmick is genetic mutation. All Pokemon can do it, but only some change form completely. The name for when these certain Pokemon change form is super genetic mutation. I have added a design to this project that is a concept for what regular genetic mutation looks like. Please remix it with your ideas on how regular genetic mutation looks and affects regular Pokemon. I will be sure to consider any ideas and let everybody know once I have figured out what it could be.