Not sure where to put this, but I figured the "suggestions" forum would be a good spot.
I had a brilliant thought while I was running through TTW's alpha version earlier today (which I must say, I've yet to encounter any problems even with a handful of classic FO3 mods installed).
Many people have complained about the time difference it would take to travel to the Mojave from DC, and I agree - land travel seems almost game-breakingly silly. It's post apocalyptic America, what are you going to do, take a train? But then it dawned on me - MOTHERSHIP ZETA...
If you make the Mothership Zeta DLC a post FO3 main quest requirement and link up the travel system via a 'teleport room', the time differential and "believability" of the travel becomes almost fluid. Add a small pre-FONV area where the teleporter goes offline Back to the Future 3 style and set up the necessary "pre-events" with some custom NPC's and BAM - no story gaps, no weird transitions. After FONV's main quest is complete, trigger our custom engineer NPC to have 'fixed' the teleporter and BAM - free travel back n' forth with no awkwardmess.
Just an idea. :)