Raiders- steal people's supplies and kill people out on the road.
Cultists- kidnap people and sacrifice them. Maybe they live in a church or something.
Cannibals- kidnap people and eat them. They can come at you on the road and try to take you.
Just different bad guys would make a unique story and gameplay. They could attack your base, you could locate and attack theres. And they would definitely need to be larger numbers for them. Maybe they could set up in a base and if you kill them all you could move there if you wanted. Have it so there could be some sort of a main villain to focus on.