I'm not exactly sure why buildable vehicles aren't possible, but I assume it's because bricks are entities, or something, and entities cannot have vehicle physics, or maybe they cannot be welded together and move, I'm not sure. But I suddenly came up with a solution.
Maybe there could be some sort of vehicle editor. Sounds like a lot of work, implementing some kind of modeling system.
Heck, now that I mention that, there could even be like a gun editor, brick editor, etc.
But the only problem I can think of is other people downloading the datablocks. Maybe the vehicles can only be made before you start a server?