As Ferris says, "You're still here? It's over. Go home."
I enjoyed Oblivion. But I must say that the after party left a LOT to be desired. Gates weren't popping up anymore, the center ofthe city was different. I had a statue someplace. People would talk about me like I wasn't the person who had just saved their asses. But no big change. More of a, "return to normalcy" than anything. The only reason to have the keep playing option, as I see it, is to finish up what you missed. NV gives you plenty of warming so you know you can do that, before you go into the last mission. It surprised me when I saw it was my last mission, but I saw the warning and back away.
Some people have suggested that they could just make it an option to keep playing in some vanilla version of the world. Sadly, if they had originally, I would have thought that this was part of how they intended you to play, and then I would have been disappointed with the world not reflecting my actions. Clearly, either way, someone is disappointed. Someone in one of these threads mentioned Dragon Age: Origins as a game that let's you keep playing after the end. Not quite true. After you beat it, it puts you back in the game, at a prior time, before the ending.
AI doesn't make for the best companions. In the first two FOs, I vaguely recall one companion that killed my character far more frequently than he killed the enemies he was supposed to be shooting at. I hate getting shot in the back by someone who is supposed to be on my side. I also hate killing someone who is supposed to be on my side because they constantly run in my line of fire. The only good thing about Fawkes was the amount of damage he could take. I used Fawkes once. Never bothered with him after that. Never got the others in any of my games.
Dragon Age: Origins doesn't have a "play after the end". It makes a save and any dlc you get starts from that "after the coronation" save. You can't do anything else; you are restricted to the party camp unless you're playing the dlc. You never go back to the game world *after* the coronation. The expansion Awakening continues the story, but even that doesn't let you back in the DA:O areas. You get a cameo by whoever you made ruler, and unfortunately one companion from the original game is available to be a companion in this game. Another game people bring up is ME2. You can play after the end, but unless you really, really like mining for minerals, nothing changes. You can finish missions, but there isn't any point. You did the main one; you don't need anyone loyal, you don't need any more upgrades to the ship, you can't upgrade or get better armor for your companions, your companions have no new dialogue, so playing after the end doesn't matter. You can get the Lair of the Shadowbroker, but even that has inconsistencies. If you wait until after finishing the main mission, you get information to give to Liara. Considering I told TIM to shove it and shut up, I don't think he'd be nice enough to help me with more information immediately. None of the other companions have anything to say about the mission or Liara, even if you invite her to the ship to continue your romance.