However I can see if you are NCR or Legion that the palette would be a wee bit thinner. With yesman, you could almost have alien invasion and I wouldnt be surprised.... because if he was the canon choise, giant puddings from outer space makes sense too.
I'd argue that an independent Vegas conclusion would have more to do post Hoover Dam.
There's dealing with the political fallout of expelling the NCR, rise of raiders and bandits, the possibilities of forging alliances or taking over by force the towns and factions of the Mojave, etc...
You could even include a Yes-Man betrayal if it is to happen.
I totally agree with OP, I have yet to finish the game and I find it annoying you would have to avoid the Main quests,
Cos when you get to a point where you don't know what to do next, You are forced to roam around looking for side quests while you have the chance to do them,
I'd really like to see the after-effects of the main quest and have my quest log less clogged up for a change
I would like to see the after-effects of the main quest, but I think for that to be visible we'd have to skip the time line ahead a bit.
And it would be a ton of work to pull of for a DLC (or any game without picking one ending).
Your other point is non-sense, though. Part of the point of continuing after the main quest is so you can roam around finding things you missed. Which is something you can do just as well before finishing the main quest.