I think what Fallout 3 could really be awesome with is some wicked unique weapon models and music replacements. I'm quite surprised no ones done them yet but I think this would be neat as:
Music from the 80s Terminator movies for exploration/combat/hud sounds and such(scanning sound could be used for pipboy)
Maybe even have some unique sounds for your character. Also more terrifying zombie ghoul sounds from the RE2 remake,
Have the machine gun model from Resistance Fall Of Man being used as a replacement
Plasma rifles being replaced by the weapon model from Doom 3
Have some Terminator focused laser weapon skin replacements.
I would love to see more xenomorph creature skin replacements for the mirelurks or something like that.
Maybe have some unique horror skins and models for the game.
Just simple ideas. nothing out of the ordinary.
Thank you for your time.