A harem mode was planned but is such an enormous task that it has been put on hold for now for the same reason as above. If the game is received well enough to warrant extra content after it has been completed it is high on the list of stuff we want to add in then.
I don't think there will be a lot of content that isn't pretty vanilla, but I can add some choices before optional sex scenes anyway. That might mean you are cut off from sexual content with certain characters though. Demon girl Maaku is dominant in bed for example so if you don't like (very light) femdom there will be no replacement scenes that don't fit her personality as well.
Scenes with the main characters are of course mandatory if you enter their exclusive romance routes unless you turn of adult content altogether. Overall those sex scenes will be more romantic, but if you don't like the idea of having sex with Kyta you probably shouldn't pursue her if you have adult content turned on.