I also thought about a rainbow pokemon but I would like all of them possible or some of them at least. I can think how badass would be some of them full of colors. And I like your idea of a rainbow pokemon would have all powers (dark/shiny...) or something like having half ability like having 450 hp and more resistence to get (burn/frozen...); but to be fair you would be able to have only one rainbow pokemon in your team and not six rainbow at the same time if with all powers or something to equilibrate because it would look badass a team of just rainbow pokemon. I would like them at least with rarity (common/rare) and not getting them just by events, events could be just for specific ones.
I also wanted we were able to get Pink pokemon just like in sidequests, not all but at least those ones we have there.
And Crystal Onix that we also have in sidequest, I love it. It could be some event or being like a legendary to find. It could be rock/ice or ground/ice.
Crystal Onix and Pink pokemon we also have in anime, so it would be really cool we able to get it