To be clear, "fully Evolved Pokémon" means something that can't further Evolve, though Mega Evolutions and BREAK Pokémon have complicated this; yeah, even though Mega Evolutions are part of the source material, still a complication the game didn't need. "Role" means recognizing the generalized functions scene in at least some (if not most) decks: opener, closer, main attacker, Bench-sitter, etc. Some roles are likely only appropriate to certain Stages, but it is mostly "Set-up attacker probably should not be an Evolution."; being the main attacker even when something is a Stage 2 should be plausible and without Evolution acceleration. In fact, most acceleration in the game is why Evolved Pokémon struggle (besides cards that are just clearly overpowered =P). Don't let them hit for damage (technically "significant" damage, but might as well play it safe) on a player's first turn. Don't give them a useful opening attack or Ability and then also give them something that turns them into the main attacker or a useful Bench-sitter.
Basics have a built in advantage of needing fewer cards, and historically this was balanced out by trying to allow Evolutions to do more but again, that hasn't really worked. So instead, make sure that a Basic main attacker needs to be accompanied by something else (probably another Basic) as an opener or else just sit there for a few turns while it powers up. This reduces the card and speed advantage of Basic Pokémon. Now make lower Stages more useful (without getting into things like Night March) territory. Instead of a Stage 2 needing to carry the weight for the entire line, let the Evolving Basic be useful in setting up, maybe give the Stage 1 a solid coming-into-play Ability and then the Stage 2 can be the main attacker or the Bench-sitter or what have you.
Yes, all of this assumes we are going to have to just wait for most of the current stuff to rotate because... well that is really the only option. We've been trying "counters" for a while now and they don't work too well. Still probably forgetting some stuff, but that's a start.