I threw in with "other." It depends too much on the system expectations, where we are in the game, and players' desires and expectations for me. For us, I don't think that there is a "norm."
I tend to put it back on the shoulders of the group and have them decide what they want to do. I'd say that 9 times out of 10, if they're currently mixed up in a race to save the X, Y, or Z, they'll invent or adopt another group with parallel goals and march onward in an attempt to finish the story.
But if that sword isn't hanging over their head, and they're still enjoying the game/setting, they might adopt an alternate storyline that they had crossed over. Maybe they want to play a band of brigands who are cleaning up and rebuilding the old fortress their old characters had cleared out back when they were 2nd level. Maybe the underdark farming village that they saved that was entirely populated by Warforged tickled their fancy.
Maybe they want to move on and play something else entirely.
My current game is a West Marches styled Torchbearer game, and it falls heavily into that second option. An incredibly deadly, post apocalyptic world, without any threat of overarching world saving story lines, since the world has already come to an end. They already TPKed once, picked up and pointed themselves in a completely different direction, and frequently talk about what they might try the next time when they inevitably TPK again.