I would like the ability to purchase and own slaves if there is any micromanagement in your home or town. Examples: Cutting wood, or mining ore. Even if the ability to purchase them isn't there, slaves should be a very real part of Skyrim.
There should also be another option for those who aren't too keen on treating people as property. Perhaps hiring someone as your servant, or something along those lines. Hiring instead of owning, however I'm not sure how things like that would go in the land of the Nords.
I actually like this, I can see a slave as a pack mule/fighter, but I think the real use would be to have them work at your house. I would like the option to treat them kind or cruel so they will react either in cowering fear or welcome their "kind master's" return.