I enjoyed the prototype, I am not a big fan of the new genre, but the fast pace nature and the ability to turn on a dime makes this fun.
Some suggestions:
What if the Ai rubbish/dump truck goes for the gold bars (whatever they are) and collects/steals them instead. These trucks could spawn less (1 or 2 at a time) it could have a load bar on top of it, and once full it will head to the exit or the middle of the map ?? or to a collection building it drives through it to unload those gold bars. Which means you have to destroy it before it does that, to get it to drop its load of gold bars it stole. It might need a few hits to do that. So you might let it steal those gold bars and reach full capacity before you attack it.
Harder levels it might have a turret on the top that fires a flame-thrower at you at intervals which you need to dodge, but it also destroys other cars. A twist could be when these rubbish trucks get destroyed they remain on the map but in black, and you can take damage if you slam into the wreck, so it becomes a hazard to avoid. However the player could push these trucks if they slow down and go slow. Which if you have several close by you could build a wall that the Ai cars have to go around. Perhaps the Ai cars cannot move them.
This could mean you could create gaps in the waves of enemies, however you still don't want to drive fast into this wall yourself. Its optional to move them. But could be a tactic when things get harder. But then you might later get Kamikaze enemies that explode on impact and this explosion might move these wrecks.
Other idea is, having pillars that come up from the ground for a few seconds and then go back down again, which you can see the rectangle or square outline on the ground so you know where they are. They all could come up at the same time. If an Ai or yourself cross over it as it comes up it will launch them into the air.
Also it would be fun to have static objects on the map that you can drive into and they become a projectile, like shopping carts or barrels. What about dropping down sticky substance that slows down whatever drives over it- this could fade away after 60s, which means if you double back you have to avoid it yourself. This could be handy for crowd control. Upgrades to it could allow you to dispense the stuff longer, so rather than a puddle it could be like a long one.
No doubt you will be thinking about different terrain/maps like a junkyard, desert, ice/snow map where driving is even more tricky for other levels. It would be fun to try and find the triggers for these, which the car falls down a hidden hole to the next map or there is a up-ramp like a multi-level car park.
Anyway just some ideas.