I know the idea of creating a Champion who enslaves others would be dicey, but I feel it's a necessary evil to show that Runeterra is a rather dark place. Making a Demon who is basically a slaver would be a more appropriate angle, IMO, since you'd expect that kind of evil out of a demon.
Would this help to expand the dark side of the lore, or would this be the same as the "no skeletons" rule?
I think you're going to have to wait for Mordekaiser's eventual rework as Domination and Enslavement are pretty core to his theme. His kit is unfortunately quite a mess right now so hopefully he'll get in the update schedule sometime soon after Akali and Nunu.
the "no skeleton" rule applies to humanoid beings (karthus is a good example, he is not pure bones.), so you could get around that depending on what u have in mind for looks.
"slaver" however could get dangerously close to Thresh's thematic.
What kind of gameplay would such a character have? We already have multiple minion-mancers, and aside from that I don't see much opportunity or reason for a slaver champion. If it's just being made for the sake of having a slaver champion in the roster and no other reasons then it would be pretty lackluster IMO.
