I really think that in PVP you should start having players being only allowed to use the cards of their rarity and below.
Like for example, if your rarity is INK, the highest card rarity you can use is INK, you're not allowed to use Invasion or Feral or Legion cuz some people might not have those kinds of cards yet.
And if you leveled up from INK to Invasion, in PVP, you'll be allowed to use Invasion cards and fight people of your rarity who also have those types of cards.
It's not fool proof but helps PVP be a little more fair. I'm tired of losing because all i have is two ferals and and some INK cards and i have to fight a player with his 2 Legion cards and 4 ferals. It'll help make it a little more fair for some people who'll have a harder time catching up.