I love this game, it's a ton of fun and the game play almost never gets old. But I do have some ideas that I think would be cool to see.
Idea 1: a throw axe upgrade ( it could be separate from the fire button)
Idea 2: an upgrade that makes you shoot both ways but halves your damage.
Idea 3: a dash ability (or something that buffs your speed after getting hit)
Idea 4: a riot shield that can block ranged attacks and buffs health but has low range and nerfs speed (you could reuse the axe projectile sprite)
Idea 5: A difficulty that doubles (maybe triples) boss spawns
Idea 6: A boomerang weapon that could pierce infinitely when returning.
Idea 7: Multiplayer