I don't know how big vehicles can get before they lose practicality, but I'd love to see some sort of battleship. In my imagination, it would have several mount points. One point would be a driver, a couple would be passengers, and the rest would be on various weaponry around the ship such as AA guns or machine guns of some sort.
Now I've never built a vehicle before, so I don't know how things work. Unmounting would have to be off to one side to cause a player to land at some sort of port (prebuilt by players). I'm not sure what other factors would have to be considered.