I'm not saying your idea is bad, but there's a reason there are no vehicles in Fallout 3/New Vegas that are driveable. Either the Gamebryo engine, or Bethesda's team (and Obsidian's) do not/can not make vehicles that work well. Even horses in Oblivion were pretty clunky.
Now then, vehicles for Fallout 4 (since Bethesda's next game, The Elder Scrolls V: Skyrim is using a new engine)? Sure thing. But at the moment I think people just need to let the vehicle idea die.
Though the first part of combining anything would be awesome, but there's also the issue of how painstakingly time consuming that would be. But I wouldn't mind things that have absolutely no purpose, like Whey Stones and Paperweights actually having some use to the game aside from being clutter.
Edit: Also a fellow user, gabriel77dan has made a DLC suggestion. Please use http://www.gamesas.com/index.php?/topic/1148483-future-dlc-suggestions-5/ thread for any other ideas you get.