Sounds like a quest-scripted version of real time settler. In fact, I would like to see there will be some quest that player has to clear some vault/building for the followers to loot important materials or furniture.....
Another interesting plot is that, as the player can join the BoS in FNV, ne may got a task from the Elder in hidden bunker to go esat to reconnect with the lost chapter there. The player will have to pick side between BoS and the outcast (or, booker a peace).
For the location, too bad it may crash with vanilla quest, otherwise the Lincoln Memorial or Temple of the union will be a good choice, it got a large flat land that is not in use
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Personal thought though, as DC wasteland is a more desperate place than Mojave, it seem more likely that a large group of refugees/escaped slaves (especially the Pitt) begging the player to take them east.......