When I was a young lad, I would play an amazing PS2 game called Rumble Racing. I think we should use inspiration from that game when designing new powerups.
Here's some examples, altered by me so that they make more sense in the context of the game:
Oil Slick - can be dropped behind your car for slip-sliding action. Self explanatory.
Battering Ram - a front-facing shield wall that batters opponents aside for a limited time. For extra functionality, maybe it could also protect against mines.
Shockwave/Pulse - a wave of energy around the car that knocks enemies away. Already in the poll at the top of the page, so it'll probably be included.
GRIP - not sure what to call this one. Drastically increases your traction, so that you can stick to walls no matter what speed you're going. Will make it much more difficult to be knocked into the air by other powerups.
Drone (not from rumble racing) - a small drone is deployed from the vehicle that guards it against mines and incoming rockets with a machine gun. The drone also fires on opponents, but its' machine gun is not as powerful as the one the vehicles themselves use. The drone is also not completely guaranteed to stop/destroy mines or rockets.
Otherwise, I am really liking the idea of the Snare and Virus weapons discussed elsewhere.