it would still be illegal just like all those mods that create mp5s, warhammer armor, final fantasy "swords", the fallout story or star wars helmets...
all these things are copyright protected and property of the liscenced holder of its rights, all those modders can be seriously lucky noone went after them yet, especially star wars stuff...
if you recreaate somin somebody else did already and holds rights to then you need to get a permission or liscence, easy as that, no if nor i thought nor my mommy told me... thats how it is
just because so far no beth modder i know of has been sued it doesnt mean that some wicked and twisted lawyer sees a chance for a quick buck and starts his own campaign, which he can...
You can safely distribute a Stormtrooper helmet in a mod, as long as the 3D model and its textures are your creation and not the creation of the copyright holder. If Lucas Arts doesn't like it, they can remove your file on a modding website, they have every right to, but they'll never bother. They have bigger cats to catch.
And just look at Oblivion's Sheogorad Isles project by Phitt. It takes place in a location found in Morrowind, but all meshes have been created from scratch with custom textures.
This said, it'd be a tremendous task with very little result. Just getting in all the dialogue together is a hassle. Plus, you have no other choice but to have voice acting or most of the players won't use it. And, the main thing, you have to use the new Fallout mechanics. Most of the skills are out, most of the perks are out, the Special/injury/DT system is not the same, etc.
http://www.fallout3nexus.com/downloads/file.php?id=4821 if you want to venture in it. I made them for those who played the previous Fallouts who wanted a bit of nostalgia and how the vaults would have looked like in 3D, first person view. But they can also be used in modders' resource.
http://www.youtube.com/watch?v=O6WYb0XMmwE