It won't happen. It would be great if it did, but Microsoft won't allow it.
Wrong, MS allows mods in certain games, but only if the developer supports the idea and takes care of quality testing/EULA/money the mod makes (sales) etc.
Tropico 3 supports mods on 360. So MS won't stop anyone, it's the developers that decide if they want to invest time/money into the mods.
Therefore I strongly disbelieve that Fallout NW will support mods for 360
If you want to make more mods happen I think you should support the developers of Tropico 3 since they've got the right idea