- Small, low-effort sprite variations for each individual enemy, so each time you fight another one, they look a tiny bit different.
- Instead of adding moderately-difficult enemies who have a few different attack patterns later in the game, like how some platformers do, I think you should make easy enemies with lots of attack pattern variants closer to the start of the game and so on. It's always fun progressing further into the expanse of the map with the promise of finding a new enemy, but adding some unpredictability to the same enemies we face every time spices the game up.
- Put a lock on the gallery you haven't already defeated or been defeated by :P
Despite the other comment, unlocking enemies for the gallery as you encounter them actually is planned; it's just all unlocked right now because it has enemies that aren't in the main game yet. I'm mostly doing this to encourage exploration and mitigate spoilers that you might get from enemy descriptions, but I'll probably include a button in the settings to just auto-unlock everything if that's what you're here for. For me personally this is a game first and a gallery second though.
I would do the sprite variation thing, but it would be a kind of a pain to keep track of what animation variations link to which other ones, so I'm probably just going to stick with one look per enemy. Stronger variations of enemies are planned though.