not really. you just go back to the main place of which faction you went under:
lucky 38 (house or yes man)
ceasars tent
an NCR base
ect.
listen to some corny victory speech,
get your reward, exp, rep and karma.
then screw off and go do what ever you want.. seems very simple to me. something like that wouldn't even have needed a DLC.
As others have pointed out, there won't be anything left
to do after the MQ is complete. When you pick a faction to back, the quests for the other three are
permanently locked out, and any quests left for the faction you back would make absolutely no sense in a post-Hoover Dam environment as they are all about getting that side ready for the battle.
As for the reward of XP and Karma, you get those anyway, although after finishing the MQ neither one really matters given that that character's career is now over.
You can
already 'screw off and do whatever you want' by simply
not committing to the final battle when the pop-up message warns you that it's the point of no return. Hell, you can even make a fresh save game if you want (and probably should anyway), let the final battle play out, then reload said save and do as you please.
It really isn't as simple as you seem to think it is, since in order for post-MQ play to make sense the various people and places would have to be altered to reflect the changes you caused, which is a monumental task at best given the number of possible combinations of results.
Not implementing such changes would just piss off everyone who wants their actions to actually matter, in much the same way as
Broken Steel negated your choices through much of the MQ by not showing the results thereof. In fact, depending on which choices you made at certain points, it arbitrarily
replaced your choices with the preset ones that came with the DLC. For many of us, myself included, the only good part about
Broken Steel is that it fixed the gross stupidity of not allowing either of your
radiation-immune companions to enter the chamber and press the buttons. As F:NV doesn't have that issue there's no real reason to 'fix' it in the first place.
If it were possible to have post-MQ play that actually made sense I would back the idea, since it would be interesting to see the actual implementation of the changes you effect, however it really
isn't possible, so I oppose it as anything less would be an utter waste of time and effort, on both their part and ours. I know I, for one, would be
mightily pissed off if, after backing the NCR, the Legion camps and outposts continued to respawn when they have clearly been driven from the region, and
vice versa.