...the fact that the game ends.
Nope.
And this thread's come up multiple times a week for months and months, I'm sorry to say.
Personally, I had no problem with Fallout 3 ending. The problem was that it ended
badly, not that it ended. And Broken Steel didn't improve things (to the point that I don't even bother installing it anymore when I play FO3. The game's better without it.)
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Open play will not come to FO:NV. Mostly because the designers said specifically that it won't. But also because..... it'd be impossible. There are too many, very different endings (depending on which plot you follow) for a "keep playing" DLC to have to resolve. It would take waaaaay too much programming to do it right. And doing it wrong..... why bother? You can just reload your last save and go wandering, in that case.