Somewhat buildable vehicles are already possible. There's two different ways to do it.
You will just need to make a seperate model for every different part you want on the vehicle and understand that when the server gets laggy we will suffer from the "my hat is floating 3 feet behind my head" bug that was a problem in the old Blockland.
It'd be better just to make every possible part on the vehicle hideable like on the minifig.
Either way we don't exactly get a buildable vehicle, we get a customizable vehicle, a bit like lego racers. You won't be able to put a house on top of your car and read the newspaper in your living room while someone drives you downtown, but you'd be able to put different decals, windows, doors, engines and accessories on your vehicle so they'd all look different. Kind of like Lego Racer, except a bit more limited. Maybe Zero Gear would be a better comparison. You could choose from a variety of door and engine designs but you probably couldn't shift one particular door design the left or right one stud.
I don't think there's a good way to let you put your house on wheels without redesigning the way bricks and physics work from scratch.
