What about a way to put boxes or other things to be shipped in the back of the cars. The added, uneven weight could affect the vehicles in a cool way. It would also be neet to see luggage or crates spilling out in a rollover, rear ending, or extreme acceleration.
That's definitely been brought up before. You could open the level editor and spawn a steel barrel and move it into the back of whatever car. Or, if you have alpha. give this a try: I lined up the box truck at the end of a ramp, and drove the stripped pick up into the back of it. I switched back to the box truck, and was driving around carrying a pickup in the back
It came close I also threw a Covet at it, to see if I could jam more in the back of the box truck (which had become very sluggish). It wound up halfway overlapping the side wall of the box truck bed, and threw the whole pile onto that side of the truck. I then repeatedly a concrete barrier on the pile until the game finally crashed; I was trying to drag the barrier around inside the pile without pausing the simulation ... that exploded the engine