Agreed with Venasour. Mew Prime, one Rainbow/Prism. Zebstrika would be near staple in Zeels, unless I'm overlooking a better Stage 1, which would make decks less Trainer based, which tbh doesn't seem unhealthy to me. If CMT fails to get everything set up by T2 it gets massively hurt by trainer lock, and it just plain loses to Take Out spam from T2 so it'd die down. Quadrakion wouldn't get hit too hard, Terrakion and Mew Prime trading KOs just means that matchup would come down to who went first, but that's what the game is today anyway.
The game would evolve to Big Basics but with backup Stage 1 attackers, at least 2-2 lines of something in every deck that can't guarantee a return KO on Mew every turn. Mew would still need Jumpluff.