I'm not sure how difficult this would be to achieve but if you entered a den of any species and they had female slaves you could have the chance to try and save them, after which if you succeed you could interact with them in a prompt to either set them free or enslave them yourself. If you decide to sell them you could do so to the slave traders or use them as prostitutes to make passive money. Other options could also be possible depending on how edgy you wanted  it to be. I also  figured this implementation could give different builds like physical, magic, prostitute relevant types of companions that are bonded to you until they are killed or sold. This could also branch out as more items are added to the game like armors, staffs, stronger weapons that you could equip to these saved sex slaves. Hell the depth of this could even go as far as giving these AI the same stats or some stats as Lona allowing them to level up depending on what activity’s you have them doing. For example, if you clear a ork den and save 2 slaves you could have 1 slave you groom into a melee fighter while the other you use for a prostitute. So while the one who is fighting for you is leveling up it’s physical and constitution and getting stronger, the prostitute is leveling physical and constitution as well but at a slower speed but might have its own skill tree that with its new levels you can choose what skills it has like Lona. 

The more I type the more ideas I come up with sorry if it all doesn’t flow together better.

You can save them already, actually. If you speak with them, they will follow you (not all of them though). If you lead them to the exit they'll vanish (and are counted as saved). You get a few coins for it, which can actually sum up to a good sum if you repeat it at an easy nest.

Sadly, their path-finding AI is rather bad, so leading them through more complex caves leads to them getting stuck so it's rather tedious.

Slave trafficking would be a good idea. Selling them to Milo would be a good mecanic.