I'd like to see just some regular weather put in. Even seasonal stuff. Rainstorms, thunderstorms (with lightning! maybe add in the possibility of getting struck!) in the summer and spring. Snow in the winter.
What I'd really like to see is a mod that makes F3 fit in closer with reality and F2: Regrown trees and vegetation (there were plenty of trees and bushes in fallout 2, think arroyo and the wilderness), and a much lower radiation level for many of the bodies of water in the game. Sure, a nuclear apocalypse is a force to be reckoned with, but mother nature is a though pot to crack, if you know what I mean.
I know the game is supposed to be designed with a complete destruction type of feel, but after 200 years, things just wouldn't be that desolate. I think fallout 3's level of desolation would fit better in fallout 1's time frame.
Sorry, don't want to hijack the thread, read the first part of the post!!