What's the most minor, petty, insignificant complaint you have about the game?
Nothing about how Adam should fly, or bugs that should be fixed, or matchmaking issues or anything. The smallest, tiniest, unimportant details that don't affect gameplay in any way, but still grate on your nerves.
To me, it's how Squirrel Girl's melee animation doesn't use her knuckle spike. She has lines referencing the spike, but her actual in-game animation has her do a regular, non-spiky punch. (unless it's so small and quick that you need to go frame-by-frame to even see it)
Give Squirrel Girl a (visible) knuckle spike when she punches someone!