Adding more NTR would be hot. Make it so the tavern girl becomes the MC's girlfriend, then he watches her suck all the big dicks in town. Of course, this would be an optional path. (This would probably be more stag, cuckolding or netorase, however way you want to look at it). The dev already likes the idea of the girls going around sucking cock, so why not? You've got the idea of breeding girls getting fucked by a million sex partners, and that one breeding girl could be a potential romance option for the MC.
NTR seems a natural, yet optional, fit for this game.
The complete opposite of NTR is Harem. So the MC should be able to get all these girls to himself. MC does all the breeding of the women (or guys if you're into that), and he doesn't share his girls.
The existing foundation could easily cater to both crowds.