I think beamng drive needs vehicles like motorcycles, construction vehicles, or more crazy stuff like that. an ibishu version of the Honda super cub would be cool.
Motorcycles would be really really hard to make driveable. You can go look at the Jawa Dandy mod in RoR but even that was quite hard to drive and didn't turn well.
An official air vehicle of some type would be really nice to have, Mitch already knows how to make a plane, and sam made a helicopter then dummiesman polished it up with some sounds n stuff. I think it is certainly possible and should be done soon.
Depends, railfanners like to see trains, and a Train would be nice to see since some people like to see them in games. Plus, a train would give them a realistic expirence of seeing what happens if you crash one.
Someone started a train mod ages ago. It was popular, but they eventually gave up. http://www.beamng.com/threads/train-system.11355/