This seems like a solid idea. Side campaigns for each faction.
I really want a Minute Men DLC to flesh them and the Gunner factions out. Why did the Gunners attach the MM? Who leads the Gunners? How were the MM established? And obviously a Minute Men campaign against the Gunners to put them and their leader down.
The Institute DLC could be the submarine Power Armor in Cape Cod DLC I keep talking about. Shaun either gives it to you while he's alive or he's left a holotape for you post-game.
BoS: More Maxson. Maybe internal politics
RailRoad: More toaster-hugging or trying to get into Glory's metallic panties?
Maybe something to along the lines of establishing a government and a working democracy out of the Commonwealth (for all factions).
Ideaurrs??