1) Marriage with multiple slaves. This also could be available after you meet certain requirements (completing certain quests, reaching after certain influence points, siding with certain factions or religions etc.) even if it isn't available from the start.
2) Being able to impregnate someone other than your wife. This feels unnecessarily limited at the moment.
3) More virgins. Only two girls (no, anal virgin only Michelle doesn't count) feels very limited and unrealistic. This could be affected by their age (teenage girls could be more likely to be virgins, and this chance could get lower for older slaves), their background (if their story mentions any experience, then obviously they can't be virgins, but on the contrary if there isn't any mention of such experience than no reason to be not a virgin), girls with Pure and Innocent traits, and corruption of both girl and city itself.
4) Just like how there is a slave market for below-average (random) slaves (Watery Eyes), there also should be an above-average (random) slave market where only slaves above a certain value are sold.
5) Dunno if this is already planned but ending(s) (yes, preferebly multiple endings). Possibly depending on choices you make, quests you complete, and factions you side with etc. Of course, there shouldn't be a reason to not continue the game in sandbox mode after completing the game or postponing getting the ending as much as you want.
I have a question as well. How do I get domestic focus for slaves?
Hope you read this. Keep up the great work!