I haven't tried this game yet, but I will soon. Looks incredibly fun!

Man on Fire: For a short time, you deal damage to zombies when you come in contact with them, at the cost of your own health.

Also, I think some sort of melee option should be present.

Zombie repellent: for a short time, zombies run away from you
Bounce: You now bounce high—real high. But you can't stop. But you can shoot down.

Thanks Half-update time!

I've managed to get the item system working pretty well . When you kill something, there'll be a 25% chance an item is dropped. 50% of the time it will be money, 25% of the time it will be ammo, 12.5% of the time it will be health, and so on. Armor pickups are also available (6.25%). I haven't yet coded the actual power-ups yet that's next on my to-do list. Also armor has been made way better (5x more effective, since before it was kinda worthless haha)

another powerup::  drain:If you get touched by a zombie you drain his health.

Well, powerups are only going to last for a certain amount of time. I already had a shield/invincible powerup planned, so a drain would just be an extension of that. However, it would make it kind of hard to program...The code for removing a zombie and recording a kill is fairly complicated and embedded in the bullet code loop. The code for checking for zombies' proximity to the player are in a completely different place and environment I could make it that zombies lose health via drain, but they wouldn't be able to die. Would that be weird?

So far I have fully implemented and coded
-Shield
-Time freeze
-Power fire (Bullets do 4x more damage)
-God mode (Shield and bullets do 16x more damage)

I also have a rapidfire power up in the game, but it currently causes a RAM clear as soon as you pick it up...lol. Gotta fix that Also I want to put in invisibility as well.

or adding a boss stage.