I think they would really just extend the boundaries, map-wise, of the original Fallout 3. Plus, add all the new quests, weapons, etc. I wouldn't want them to fill in the buildings or add new ones that weren't previously on the map. That would ruin the experience and I wouldn't like it at all.
"Oh...look....a hotel........I coulda sworn this was just a patch of rubble a couple days ago."