Lets say you went with Ceaser. Maybe he reopens lucky 38, maybe he starts demolishing it. Maybe he turns it into his vegas headquarters. There are tons of things they could do because there is already a ton of crap to build upon for ALL factions.
Yes, there are "tons of things they could do".... and all of those things would need to be programmed, for a "continue" DLC to make any sense. Ok, you sided with the Legion..... well, based on other things you did, which towns did they destroy? Is Novac a smoking ruin? (Whoops, sorry about anything you had in the motel) All the casinos are burned to the ground, all the people in the outskirts are in chains and/or on crosses. Almost every civilised area is missing most of it's NPCs. (Probably blows most of the sidequests you haven't done yet, so much for "but I still had stuff to do").
Or the DLC is before all that finishes (since it would take time to subjugate everything). Which means you need to program all the NPCs to be panicking about the oncoming horde. Legion attacks brewing up all over the place, new quests to help or hinder, new voicework for everyone (or it'll seem stupid because no-one's talking about the disaster). Ok, we're almost at the level of an expansion or sequel game with this one..... certainly beyond the scope of a simple DLC.
And that's just for *one* of the main ends.