Just a few ideas off the top of my head :
= RPG aspects =
* Improvable skills that very slightly improves the magnitude+stamina/time efficiency of various actions per skill level. (Scavenging, hunting etc.)
* Once skills hit a certain threshold, they'd yield a single trait point that could be invested in a multitude of improvable traits
* Traits shouldn't just be a flat increase to boost skills. For instance - a trait that allows the player to have a 15~30% chance of disarming an attacking melee enemy.
* Body armour that soaks damage via a percentage
* Hard cap for base stat improvement but the MC could be further enhanced through skills, traits and equipment.
* Damage types & resistances (Like fire, pierce, slash, blunt etc.) to make combat and armour a little more varied.
* Status ailments (Burn, knock down, bleed, poison etc.) & related combat actions to improve turn-based combat
= Gameplay =
* Expanded bounty system which ranges from the kind of bounty hunts we have now to slightly more complex repeatable multi-stage mini questlines which involves hunting dangerous individuals, minor gangs etc. with better rewards
* Potential companions in the form of unique NPCs or the randomized slaves you turn into guests with their own stats/traits/skills.
* Certain inheritable traits that improves the character and their base value
* Base building extends to the point of establishing & maintaining a small settlement as a means of late-game money/resource sink.
* Maybe have the base absorb nearby structures like the Farm once it gets big enough
* The player could get their arses raided by hostile factions or minor independent groups once they hit a certain point of the game
* More vocation types for guests and slaves in regards of base building with the end-goal of automating resource production and other shenanigans.
* More vocations that are exclusive to either guests and slaves to further set them apart with the former having managerial/leadership/defense roles etc. and the latter performing menial labour etc.
* Guests and slaves with high affection+submission to the MC should have slightly higher productivity (10~20%?)
* Guests should be better than slaves in terms of learning speed, productivity etc. but cost more to maintain
* A loyalty stat that serves as the alternative to submission?
* Able to commission, upgrade and maintain a variety of Mad Max styled vehicles that provides a variety of benefits (Namely shorter travel time)
= Flavour =
* Potential slaves should have higher base beauty+higher beauty threshold dependent on the event and location the player encounters said girls
* Preset personality types that is randomized in slaves/guests so they wouldn't all share a single dialogue tree
* Various different approaches to starting a battle via the game's various events - like sucker punching one of the enemies in the random club event (outright knocking the poor sod out cold or just stunning+damaging them through a strength check) or using the bow to strike down one enemy from afar in the event where you "save" a random girl etc.