So some thoughts after I've played for a while.
1. The flame gun is VERY weak and slows you down SO much that I cannot make it viable
2. My favorite weapon is the shotgun but all the 'extra projectile' upgrades are trash for it maybe add 2 projectiles for the shotgun.
3. I have played for while and I am super good at this game so take this with a grain of salt: the boss that traps you in the red box thing feels too hard. I've played for ~4 hours, (I love this game if you couldn't tell), and gotten some great synergies and amazing runs and never beaten it. Maybe make the box bigger or lower the boss's health or speed.
4. From what I have seen there are no spread reduction/accuracy upgrades. A small spread reduction/accuracy upgrade could go a long way. It could be a scope.
5. There is no active healing, and the only passive healing, the regen upgrade, is locked behind the giant upgrade, which can be run ending. Maybe you could add a small chance (like 1-2%) that the enemy's drop a heart or every 50-100 monsters killed spawn a health dropping enemy. Or just add VERY slow starting regen, like 1 heart every real time minute.
6. I would love a Retry button when you die because exiting to the title screen and entering again is kinda annoying.
All just opinions not complaints.
Thats all, I love the game, keep up the good work.
-Cody