I mean it's not entirely out of reach, like recently someone did a gun mod for new vegas which put a cod zombies weapon into the game. And these days especially fallout modders make some ridiculous stuff.
I think it would be wiser to just make weapon mods, a few quests and maybe some clothing not to make an entire game. I mean it would be awesome if they could do that.
Hell I remember someone made a mod for Fallout 2 which let you play the game a littlepip.