Nice game!
IMHO, the background is pretty distracting, would be nicer to just have it blank.
Ideas: 100 hp, hits hit you for 40, 1hp/5sec regen, and enemies have random drops. Drops could be 10 hp heal, bullet bouncing shield for 3sec, etc... Additionally, every 30s a dice is thrown on the blank space on the side of the screen and an event happens according to the roll, for example, 3 new enemies spawn, you get 5 points, etc.
