So I think with the Fallout 4 the lore and choices were a little lacking I would like to see that change with dlc. They could add factions from all over the waste of America being brought in by some group of people telling them to go to Boston. It would give people a taste of what has happened in the rest of the Waste and could give an interesting look into what happened in the other games (what choices are actually cannon from fallout 3). With the factions that come in and give you a taste of what the world looks like you can decide whether to side with them, integrate them into your faction, destroy them, make a treaty, or force them to leave the Commonwealth. Each faction you side with or against gives you unique abilities for your settlements. For example if you side with a slaver faction from the Pitt you could sell settlers from you settlements for caps or supplies from the Pitt.
I know what you are thinking oh no more settlements, not really maybe you could just give them supplies and help, then the faction just builds their own structures and grow their minor factions. I would like this to be free maybe Bethesda could use a dlc like this to see where the community would like the series to go to next.
The faction telling people to go to Boston could be a villain trying to orchestrate a grand plan or a group just trying to bring people together.