I'm not sure if this can count as entry but here it goes:
I can't think of any in-game mechanic that I'd like to add because I'm totally okay with how Splinterlands currently is. What I want as an addition though, is the battle pass system that is similar to Dota/Valorant. It is basically buying an item that is worth >$10, then you get to unlock additional items after that by levelling up and doing quests based on that.
It would be fun to have something like that, which could serve as an additional daily quest where we could unlock cosmetics or one-time use ECR regen potions.