when i saw another game "Dying Light(this is another FPS game with RPG materials)" i think some design ideas is nice for Fallout series games.
1. allow player to play second round storyline campaign with his/her character's level and skills and perks and all items he/she carried after finished first round.
2. add "grappling hook" let player can jump to higher places and player can use it to protect himself(herself) when he/she fall from high places.
3. allow character to custom weapons or equipments by using trash materials, apply some elemental damage into weapons(fire, freeze, electricity, acid, poison...etc).
4. allow character to ram his/her enemy, or add some grab moves like shoulder throw.
5. add two-weapon fighting style, like holding 2 guns or 2 blades to fight.