here's some ideas I have for the game upgrades
A circular wave that happens every certain amount of shots. For example:
fire 8 bullets around per 8 shots
Combos increase value of coins (like 1.5x combo is double coins or smth)
Modes you can add:
Glass cannon
makes you only have 1 heart (even if you buy +1 heart upgrade) but gives 100% more damage
Shield Force
makes you have 0 hearts but you get 5 shields that cannot be healed (you can increase hearts but it gives a shield instead)
Fragile
1 heart with no buffs (and no increasing hearts)
Tank
5 hearts but -50% damage (hearts can be increased)
Bounty
Coin drops increase the more enemies you kill without getting hit (stacks between rounds too)