I think it would be a good idea to have a separate section for actual downloads vs. ideas or mock ups being posted. This way they aren't mixed together.
I think a RoR rule should carry over... To start a showroom thread, you need to have something to show in game. Whether it be a n/b chassis, or a mesh in game.
Don't we already have this, sortof? In the community/unofficial showroom there are pretty much only unfinished mods as far as I know. In the .Drive showroom there are finished mods with downloads... Right? That said, I wouldn't mind imposing a rule like Dkutch says, where it's only allowed to have a thread once you have in-game pics...
If you're a complete ape when it comes to getting things in-game (me), it's nice to have a thread so you can ask questions. There's no way I can get my car in-game without help, even after poring over the Wiki. I mean, one thread for a bunch of questions is better than a thousand threads clogging up Questions and Support all titled with help plzzz how do i get beems for sketchiup car i make. In regard to the question posed by cml1of4, they already are separate. Reiterating Mythbuster's point -er, question-, vehicles and terrains in progress generally live in the Community Showroom, whereas completed projects and downloads live in the Drive forums.