I have some ideas for the game
Classes -
Sniper, maybe 0.30 attack speed. 75 damage. maybe 3.5 speed. shoots with big gun and its range is the entire screen.
Mage, low health (50?) but high speed (6?). It shoots a wide ark of piercing blue magic (20 damage?).
Ninja, range melee hybrid. Shoots shurikens from a distance, but slashes up close. (I dunno what code you are using, but if you are using python then maybe it could be an if distance to nearest enemy < some number, then shoot shuriken) 5.5 speed, 30 damage, and maybe every 5 seconds it dodges an attack.
Infantry, stabs with a mid-range spear. Slow speed and attack speed, but high damage and health.
Skill tree -
Maybe their could be a skill tree where you can upgrade your fav character. Maybe increases to all the stats, then at the end of the tree you get a bonus like an extra attack or chance to dodge attacks.
I think it would be cool if at level 15 weapons got a mega boost and changed the weapons completely. Like the scull would transform into a full-on skeleton that still shot projectiles, but also maybe swiped with his hands. Or the bubbles combine into a big bubble that shoots big explosive bubbles. The fire shield transforms into a constant inferno around you, and it gives some damage reduction.
Also maybe some enemy enhancements, like archer skeletons, maybe some minotaurs charge at you, maybe some wizards
These are just some ideas, this game is super fun and I would love to see it expand. You have done a great job on this and I love playing it.
(and I had to mention it- multiplayer. Maybe 2 and 3 player co-op. Like maybe I know this would be super hard to pull off, but this would make it a little bit better)