Although it may seem like a bit of a silly, power crazed idea, I would love to see some Monster Trucks simulated. I personally love the rather over-the-top-ness of them, and if nothing else, it would be a great way to test and expand on the extreme end of the game engine's physics.
I'm very much looking forward to the rally expansion, and also like the idea of the off-road trucks in an earlier post.
Hmm.. I see a theme emerging!
I guess I'm looking for a bit of variety - I remember back when the very first demo came out, that one of the mission statements included comments about plans to include all different types of vehicles.. buggies, karts, 6 wheelers etc (unless my memory is lying to me heh). So far though, there hasn't really been much of that at all, and fair play for catering to the proportional target audience and all, but I would love to see something rather more experimental than what we have so far.