I think it would be an amazing idea to let the user be able to fix up an abandoned Car/Motorbike (preferably a Motorbike)and use it as transport within the common wealth.
I think this would be a good idea because adventuring around the huge map in fallout can be more of a chore than an enjoyable experience due to random enemies that can easily kill you, Yes as you get to a higher level most of the enemies become easier but when you are out of ammo or stimpacks and you need to travel a long distance it can make you less confident about the adventure and make it more stressful (especially when there is no autosave).
The vehicles can operate like the power armor depending on fuel to keep them running and having to repair it if you crash them. They should also be able to be upgraded like adding spikes to the front or adding bigger saddle bags to store things in.