A milf harem would be interesting as you mention, although you could also do harems to take advantage of the dom routes of some girls, like a bourgeois harem (Aletta+Cassidy)+(Cherry) or dominatrix (Camila+Sasha), even an advantage of the harem burges is that it can work in the submissive routes (High class harem). the only bad thing about a dominant harem is that maybe they should do b versions for Cassidy's story, although I think some scenes would work regardless of whether she is dominant or submissive.
Another idea that may not be very popular is a fujoshi harem (Jack + Emma), I mention this one because I remember reading somewhere that there were plans to make Jack a love interest of Mike, personally I don't consume yaoi content but I know that there are those who would like it, and perhaps a harem would make there more reasons to add the character as one more romance. (maybe it would work better with Bree MC, and instead of Emma they could add Scottie)
Another harem option would be to resume the crazy harem (Kylie + Alexis) and maybe join it with the sports harem to make a Taming harem +
and finally... geek harem, (School harem+Gamer harem+minami+Emma)
Lastly, I must say that this is one of my favorite games, I hope Andrealphus games keep up the good work and succeed in love & sex: third base
