I have a few suggestions if you ever extend the game:
1: start with less HP, but there is a HP max increasing powerup
2: make the coins just a little bigger, also five and ten coins for later game
3: make the crates able to be destroyed with bullets
4: bullet damage upgrade
5: bonus points used in a menu shop to get permanent upgrades (like a third weapons slot, unlock grenades, increase starting stats)
6: i love the reload speed upgrade, but it seems to cap out on all weapons, also it is just a little OP
7: make the shop chests get more expensive over time, but get better weapons over time
8: rocket launchers with super long reloads, but explosive projectiles
9: Make the difficulty reach a few new tiers, but only if you are on specific floors or higher (floor 10+ is nightmare if you get late enough, 14+ is Titan tier)
with these higher tiers add black (Mine layers) and silver (Stationary enemies that start spinning firing bullets)
10: more enemies!
11: damage powerups
12: Large rooms (bigger with more enemies)
13: three new room types (chest room with free chest, really rare, large shop with 3 chests, and a actual shop which shows you what you are going to get, but has higher pricing)
14: more chest types (gun type chests (gives from a category), higher and lower tier chests, mimics)
15: a whole bunch of new gun types (like a laser, a homing missile, a 8-shot, a leech gun that slowly fills hearts, a golden gun that takes coins to fire, triple and quadruple shot guns)
16: improve the speed up powerups, and get rid of the annoying side drift issue
17: rocketier enemies (T1 with regular rockets, T2 with slower, but homing rockets), ghost enemies (invisible til they get close), creeper enemies (reference to minecraft, blows you up like a regular creeper), suicidal enemies (charges in and explodes), laser enemies, and more!
18: multiplayer options (like a LAN game where you can play with friends on co-op, vs, and any other modes you want)
more suggestions coming soon!