Hmm... Well, slavery is always a useful tool for bad guys. (I've always wondered how The Railroad would react to groups that are enslaving [i]people[/i] instead of Synths, but F4 decided not to deal with that moral quandary, I guess, which could be interesting.)

But slavery is always an indication of strength and power exercised over the weak. It's a manifestation of the strong/smart/clever displaying their superiority by physically imposing their will upon those whom they view as their lessers. The slaver sees himself as simply "better" than those who are enslaved, and thus it is the slave's "destiny" to remain such, serving those "in charge."

Any faction based on slavery must necessarily lack in humanity, else the zealousness of the faction's members won't make sense. For the Legion, their warriors essentially compete for upper rankings by their combat prowess. The strongest are leaders because they "deserve" to be, as shown by their sheer strength. Such is why the Khans in the Mojave, or at least their leadership, found Caesar's tenets so attractive.

If we were to "reformat" our Khan off-shoot into a group dedicated exclusively to social darwinism, a group that jacked themselves up on custom chems, increasing their strengths regularly beyond that of normal humans, they could be a force to be reckoned with, using their strength to absolute conquest instead of simple survival. Knowing of the formidable factions in the former United States, they instead have been growing in the Canadian "heartland," taking over settlements as they go, and forcing them to submit.

They have enhanced themselves with chems to the point that their warriors can go toe-to-toe with Supermutants and even BoS Paladins in power armor, dishing out physical damage that should break a normal person. Perhaps they wear "Bane Masks" of their own (or some other kind of device) which delivers chems at precise doses to increase strengths and reaction speeds?

In this kind of group, scientists and doctors would also be highly-valued, in order to provide stronger chems which in-turn would create stronger warriors. Thus, they could be kidnapping scientists and intelligent individuals for this purpose, which provides other narrative entry.

TL,DR -

Turning a Khans off-shoot into Bane-esque chem maniacs who utilize might to subdue any foe.