As roleplaying gamers we invest alot of time and emotion into these worlds, and to have the end of MY story and roleplaying experience dictated by a designer is quite frankly retroactive and not proactive gaming.
...You're kidding me. Do you know how many of the universally-acclaimed roleplaying games out there have firm endings? Baldur's Gate? Planescape Torment?
Fallout? More recently, Dragon Age: Origins, and NWN2/MoTB?
Do you know why this is? Because a closed end allows you to tell an actual story, rather than what amounts to digital LARPing ala Oblivion. A firm end allows the developers to actually have a plot with gravitas and impact. It's not "retroactive" in any meaning of the term. How it's done in all Fallout games with the exception of Fallout 3 is, in fact, proactive, in the sense that the player's actions actually
matter. The player, through proactive action, can actually execute changes upon the wasteland. Changes which you see in the ending slides.
Compare Fallout 3. The player has no meaningful choice. They can put the virus in the water or not, but with Broken Steel, that has no real impact. Nothing the player does in FO3 matters. It
can't matter, because something as important as bringing clean water to the wasteland would require totally redoing the game world to account for the cascade of changes. So what we get is unsatisfying half-measures that do little more than place a few water barrels in the game world, because the status quo has to be maintained since anything more involved would take too much time and developer resources to make. The gameworld doesn't react to the player in any but the most superficial manner.
In NV, the endings are potentially even further reaching in their consequences than they should've been in FO3. Seeing a half-assed Broken Steel-like DLC would immeasurably cheapen everything. Suddenly you didn't have an impact on the Mojave. All those NCR guys you kicked out in the house/yesman ending are still there. All those Legionnaires you massacred in the NCR ending are still in the fort, just waiting for you to show up so you can farm them for ears and legion denarii.
Emergent storytelling, which is what it sounds like you're trying to refer to, simply doesn't work well in a roleplaying game, particularly one with an actual narrative rather than a bunch of MMO-esque events. It works in a game like, say, X-Com, because there's no real story. It doesn't work in any sort of RPG, because every RPG with the exception of Diablo and its clones are known for their storytelling.
So don't try and paint NV dropping the worst parts of FO3 and Oblivion as some kind of step backwards. They aren't.
I thought with FO3 and Broken steel that this closed ended and closed minded type of developing games had finished. Its sad.
The only way what you're asking for would be remotely feasible and not svck epically is if people perfect procedural generation of content and, particularly, stories. We're a long way from that.
It was never the basis, but Bethesda applied and failed, there is nothing to after the main quest
One thing I want to say is that without reactivity from the world, "freedom" is pointless. Without consequences, your actions are meaningless. NV gives you consequences for your actions, making them meaningful. Fallout 3 does not. In FO3, every action you undertake is, ultimately, pointless.