I have a few suggestions for the game but one in particular stood out from all of the others mainly because it's uncommon and/or never been spoken of. My suggestion is to add a pokemon that has only been played in one episode out of the entire pokemon series to be in the game. Just like how there's a Red Gyarados, I believe there should be a Crystal Onix.

Crystal Onix is an extremely rare Onix with crystal like features and because it's made entirely up of crystals it should be a rock and ice type. In the anime it also showed that it was weak against fire but immune to water. Overall, I suggest Crystal Onix to be implemented in the game but because of it's exclusive rare appearance in the anime, I also suggest for it to be an event or quest but players may only encounter it once. Tell me what you think.

There's a lot of pokemons in the anime that has been made different to give some "specialness" into the episode, which sometimes also contribute to make said episode do less sense. These never appear in the games, mostly because it doesn't really serve a purpose. The Red Gyarados, however, served the purpose of introducing Shiny pokemons to all players.

 

In PRO, we have two kind of "special" pokemons. The first kind is only used by NPCs, which I've seen only one special case outside of Mega evolutions and Primal legendaries. The second kind are alternative forms you can find with a low chance, mostly for events. They are not limited, and so one person could have multiple Crystal Onixes. I don't think it would really serve any purpose either to limit to one per player, especially when it's a perfectly good pokemon to throw into the economy.

 

Another thing you have to think about is that Onix has an evolution. This means that there would also be a Crystal Steelix, which has not been a thing in the anime. That might sound a little less special to you. At the very least, if we throw an event for Crystal Onix + Steelix, chances are we're also going to see Crystal versions of other pokemons. Or maybe not just Crystal, but also other kinds of rare metals like Ruby and Emerald colored pokemons.