Lets say for example, you have a 4k CP Mewtwo that you spent a lot of dust/candy on
But you can’t use him for anything below Master League
The way I I’d suggest to solve this is that the game will automatically delevel your pokemon so that it is just below the CP cap. So your level 40 mewtwo will simulate a drop in levels just for the duration of the PVP match
That way Casual Carl and his ragtag bunch of underlevelled pokemon can fight on a relative even ground with his buddy Hardcore Harry and his immucalate army of 90iv level 35 mons.
Harry does not need to sandbag himself by digging in the bottom of the barrel and he can benefit from having choices, so the only advantage he will have is the span of his selection