Upgrade: Tiny person
Quick summary
An upgrade that will make your player small for a short period of time.
Why it would be valuable
It would make it harder for the small player to be hit by bullets, giving a different kind of advantage in dog fights. Anyone could choose to buy it so it should be about as balanced as any other offensive or defensive timed upgrade.
Proposal
When the player buys the upgrade they shrink to some arbitrary size (like 50%). This would make them harder to see and harder to hit, they would have an easier time dodging. It would be an alternative defensive upgrade to, say, the shield upgrade.