I'd love to see mech combat mods. Due to power armor acting more like a vehicle than a suit of armor, programming wouldn't be nearly as difficult as it is in Fallout 3 or New Vegas. We would also have far greater customization for weapons, since we were confined to only three mods in New Vegas. The huge array of different options for housebuilding would also make custom maps/building mods that much more varied and interesting.
"They asked me how well I understood theoretical physics. I said I had a theoretical degree in physics. They said welcome aboard."