Not really much of a big suggestion but I'd like to see the subway system, the one you use to travel from the Mojave to the Capital wasteland look something like the Presidential Metro from the Broken Steel Expansion and like when you use the train on that level. And perhaps if you start in either side you have to repair the train system like in the actual Broken Steel mission.
You can see the train in action at 7:21 in this video: http://www.youtube.com/watch?v=lLHmszZKoO8