Apologies if this has already been suggested, I couldn't find an easy way to check and there aren't currently any mods out there that seem to incorporate this.
Unlike previous Fallout games, F4 doesn't lend itself as well to evil characters and the whole slaver system seems to have been completely removed. I'm surprised that they didn't gear settlements around this previous theme so that the player could capture slaves to use as "settlers" and some evil variant of the Minutemen.
Hopefully, someone can create a mod to bring back the slave collar (they could even add another spin to it for good players to free slaves in raider camps in future updates if they want). Thought perhaps the dog collar would be obvious choice, though not sure it's possible for human characters to wear so maybe the bandanna instead or sack hood?
The player could either capture generic NPCs as slaves and sell them to a slaver raider camp, or use them in their settlements. Personally, I think this would make settlements a lot more fun, especially if you can punish them or the defences are as much to keep them in than it is to protect them