With all the rather complicated Trucks and Trailers I would love to see a simple heavy Box on Wheels which doesn't drop the FPS and adds weight to Trucks.
Does Realism Matter? I basically am an advocate for more options concerning weight. And having weights shifting about in a trailer would be interesting I guess.
While I'm not necessarily a fan of the "box on wheels" idea (assuming you mean a low-detail car), I do feel the game could do with some simple heavy cargo props. I suspect the devs have thought of this already though, and they'll probably crop up in the game at some point.