During the mission "Out of the Fire" you find Jake Finch being ask to kill an innocent settler. No matter what you say the Raiders become hostile and attack you. Instead of talking you could instead kill the settler and Jake yourself failing that mission and starting up a new one, lets call it "Trial by Fire". Then Slag (Forged leader) can open a new dialogue where he asks you to become Forged starting a new series of quests as a Raider.
This is just one example of how such a DLC can be integrated into the game to allow you to play as part of the currently unplayable factions. Other factions like the Triggermen, Gunners and Children of Atom could also become playable. Post any thoughts, questions or suggestions below!