Ghouls would be awesome, there sprites are already used in-game only thing is lather coats/combat armor won't show up on the player when they equip them. Of course they would be completely immune to radiation
Maybe Androids? There sprite would look the same but they would have different stats then humans, like a weakness to energy weapons, more sight range and also faster health/limb regen. Only thing is Androids will be OP.
What do y'all think?