We build settlements to make adhesives, have a place to store stuff. Buy/sell stuff. To call a home.
Settlements are an example of self-producing consumeable content, because we reap what we sow and have fun doing it.
A game with a finite amount of consumable content will eventually reach the "when all is said and done" stage, where you question the point of exploration and self-improvement.
The Dilemma that any powerful entity faces is being bored of not being challenged enough, therefore why be powerful at all in a game if that's boring?
So, what's the solution?
The player, like Yoda, is a thread or constance that binds great episodes of saga together. Settlements, like lightsabers, are a tool towards a self-created destiny very personal and specific to it's creator.
Therefore creating a DLC involving the progression of settlements towards a greater goal is not only sustainable entertainment (read self producing content...), but also offers an opportunuity to make the choice players have made in the commonwealth to matter more then just having eliminated venues of interest (like destroying whole factions in favor for one).
For instance, Institute fans can create settlements with institute tech. Teleporters, lots of aparture lab white, and produceable synths to supplement the work force at settlements.
Or Brotherhood Fans can create airships, and supply lines with vertibirds. Brotherhood tech, powerarmor production, and all.
Railroad fans can create underground bunkers, underground settlements, stealth tech and supply "runners".
Lastly, all factions ought to be able to build a colony ship for a settlement on the moon, which opens a moonbase storyline.
Ya know?