Hmm.. Sounds quite interesting. Why not have the Thruster as an unlockable ship after you do certain things in Zombies mode, such as: Kill 4 zombies in 1 clip of a cannon, run 5 perfect laps in a row, etc.

Also, why not have just a single barrel cannon that can be upgraded by a perk to double or triple-barreled, or make Zombies in the later rounds have weapons too?

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Different perks that might want to be added:

Extended Ammo: For every Cannon pickup you get, you get 45 rounds instead of 30.

Battering Ram: Your rams deal 4 times as much damage.

Juggernaut: For 15 seconds, all weapons deal double damage.

Lucky Draw: Have a chance for a Zombie to give you a random weapon or a random upgrade every time you kill one.

Double Down: You can have 2 weapons at once (Use a key to swap weapons?)

Second Wind: After you die, you can respawn with 30 integrity remaining (1 time use).

Deep Pockets: Gain a random amount of extra cash every time you start a wave or when you kill a zombie.

Double-Barrel: Your cannon shots are now double-barreled rather than single barreled.

Triple-Barrel: Your cannon shots are now triple-barreled rather than single or double-barreled (Requires "Double-Barrel" perk).

Detonator: Your cannons turn into very powerful miniguns (Like on the Qirex Prototype, but reloadable like the Detonator ship) that can reload. Lasts for 30 seconds. (One time use!)

Last Stand: Your craft is packed with explosives that will explode when your craft is destroyed, earning you some last-second points.

Any of these sound good?