I think we all know that Fallout 3 is a dull, unrealistic game where "It takes over 6 headshots to kill someone", you "Don't need to eat at all", and "There are no consequences for your actions". Well, I'm going to provide a few mods that'll make Fallout 3 an actually fun game instead of a player-is-a-god-that-can't-kill-anything game.
Primary Needs: This mod adds in "Primary Needs", food, sleep, drinking and the likes of it that makes way for more immersive gameplay, but where are the headshots?
Hardcore Mod: This mod makes it to where stabbing someone in the neck will actually kill them, shooting someone's head will be deadly, and the legs/arms hurt when you shoot 'em. Oh, it also increases the base amount of blood, as well as the damage rate of weapons.
Sprint Mod: This mod adds in the Sprinting feature to the game, where (With Fallout Script Extender) holding down the run key will make you move faster, your head will bob, and your weapon will be unequipped (Unless you have the "Charge" perk).
These mods will most likely make your game immersive, full of awesome gameplay to come.