Items

Basic small/large health drops:

Green splash around player
Health bar animation Basic damage multiplier:
Red aura around player
Damage multiplier icon Weapon multiplier
Increases the number of weapons you have by 1 Poison
When your bullets hit an enemy, they also do tick damage?
May not be worth it if enemies only take 2-3 bullets to kill anyways Heat-seeking missiles
Bullets automatically seek the nearest enemy Ping-pong
An outline appears around the screen
Bullets will bounce off the screen and may hit enemies multiple times

Enemies

Health increase over time? Basic tank:

Slow moving
Triple health
Lobs explosives Sideswiper
Fast moving, straight lines across the screen
Icon indicating that sideswiper is incoming and which direction they will go
Audio alert Slapper
Spinning enemy that can slap you crooked. For a short period of time, you will be pointed at an angle.
Animation to show that you are about to straighten up. Rippler
Shoots ripples of energy that is unavoidable unless you dash through it Loot goblin
Unpredictable, quick moving enemy that does not attack you.
If you kill it, you will get an item drop. Boss
Wind ability: Pushes you a little to the side
Ability 2: TBD
Ability 3: TBD

Dash ability: Invulnerable to projectiles / enemies when dashing through them

Transition between calm music in main/game over menus to action music in the main game.
Balance audio so there is no clipping, but the game remains loud.