I'd want to rebuild Nipton and turn it into a nice small town, with a general store, clinic, tavern, etc. but not all scummy like it was before.
If you are on PC there's someone working on a mod for this.
Also having thought about it, the only places I thought didn't need a lot of work were Jacobstown and Goodsprings. These two towns seem to be in a good place were only time can make it grow. They lack quests, though that's a problem for a lot of towns. Novac and Freeside I felt could use some quests to build them up some more, but both were places with plenty of things to do and I'd prefer other places get up to speed over adding more to these places.