In my personal situation, I'm looking for something really interesting or something that will extend my gameplay hours.
Well, my quest mod "A Trail of Crumbs" does not actually add anything new to the game (other than the marked quest, perk & bunker for completing the quest) but it will guide you to 10 locations that many people rarely if ever go to in the FO3 world. It finishes in the last 4 or so stages with some of the most epic combat in the game. It's really quite tough in spots. If you go through it slowly, explore the areas thoroughly and do some looting while you're there, it can easily add 4 or more hours worth of play time to a newish game.
http://www.fallout3nexus.com/downloads/file.php?id=11283
Also try "The Cube Experimental" (One of the BEST IMO)
http://www.fallout3nexus.com/downloads/file.php?id=6270