This goal in game does not have to be to save the world, has done it so many times its boring, and yes you can save the world by stopping someone from using the rest of the nukes, this might have no impact on the world except the ones who knew about it will like you a lot better. If could still be an awesome endgame but does not have any effects.
It could be two factions fighting over control, result would be something like Skyrim civil war, some town leaders and guards replaced, the losers driven of and work more like raiders now.
Else I agree, I did not finish Fallout 3 until broken steel, it would not fit my pretty neutral / shades of good character to kill herself in an idiot plot.
Finished NV mostly as I was pretty bored of fallout at that time
Now the last point, its probably easier to sell DLC with an open ending, Many people will finish the game and move on, feel like an DLC is a bit like an follow up to titanic