Are there any mods out there that change things so much that it seems like a different game? Such as making a mod that makes the game into Fallout 1 or something like that? I'm guessing no, but I thought I'd check.
That's quite an excellent list. Makes me envious of FO3 that it has such a developed repertoire of mods, and FONV doesn't (quite yet). But then again, I guess that might all be remedied with time... =)
Apples and Oranges, really, as far as mods are concerned between the general quality of Fallout 3 and Fallout New Vegas. I think New Vegas mods generally fit in better with the game and enhance the experience, while the best mods for Fallout 3 almost have to completely alter the experience and themes.
Anyway, back to the subject at hand and make Fallout 3 a completely different game.
Well, what sort of thing did you want to make into?
Obscurum - Natural Selection does this by turning the game into a weird survival horror type game by disabling pretty much everything and adding in weirdness that's up there on the scale of the psychic mutants in Metro 2033.
For me, however, zombies have been the word of the day and I've found myself toying with a Resident Evil/Fallout type of crossover:
Zombie Apocalypse adds in spawning zombies in the exterior areas and also possibly in Point Lookout. If they kill a human, that human turns into a new zombie.