cool game! but I have some ideas that you could put into your game.
1. upgrades: more health, health Regen, reduce cooldowns, life steal, ect. (most of the problems why I die is from not getting enough health; the little bit of health bosses drop isn't enough to help me live.)
2. new slimes: green slimes: shoots projectiles if you get hit you take damage and get stuck for a sec, red slimes: slow and tanky, yellow slimes: small and fast, but weak. for new boss slimes? (idk what to call the other type of slimes) could be like the normal slime colors but they gain a new ability or quirk, could have a boss slimes at the end of the game
3. new maps: sandy desert, the swamp, the slime battle grounds, the slime palace. could unlock different slimes the more you progress the game. could collect different materials depending which map you play on. (like you could collect wood if you beat the wooded planes) could have different music depending on the map.