If I can make a suggestion, don't forget the "harem" fetish
Open relationships rule! I think NTR refers to cheating proper, but what if there were a way to develop a trusting relationship with your partner NPC, where both agree to have an open relationship as long as they don't lie about it? Could be a specific path to dating one or a few NPCs, could be a development of a cheat, could be an option presented to a sissy MC that can't satisfy gf (although the latter case doesn't appeal much to me, feels just like another form of humiliation).
A second suggestion regarding the dom/sub dynamics: most games make it impossible to play both in one playthrough - you're either sub for life, or dom for life, and the other side of the story is barred to you. You mentioned in another post that you want to make it more dynamic, and I understand that from games such as Hornstown, where events and choices give or take sub/dom points, so in the course of one playthrough you can shift between both. The thing with Hornstown is that it feels, perhaps, "too free". It's hard to strike a balance between player agency and some amount of enforced content. Personally, I feel that total lack of player agency, forced NTR, chastity don't strike a chord at all, they're just frustrating. BDSM themes in games usually assume that a submissive MC is just submissive, forgetting that there could be an interesting "brat" role, where the sub cheats or defies the dom (and gets appropriately punished/rewarded).
There's challenge in breaking apart the whole "binary system" without falling into complete indefinition. Apart from sub/dom, there can be a switch; in relationships you can have monogamy and poligamy - but I find open steady relationships more interesting than uncommitted promiscuity. Although a "harem" between a few close friends is interesting, too - instead of being noncommittal, it could be vowed by all parts after the story brings them together in the right way. In sexuality, going from purely str8/gay gameplay to bi gives the player more freedom, but can also take some interest out of the game. I find "bi-curious" more interesting than bi proper, as instead of being free-spirited and agreeing to anything, the player can struggle against their reluctance - breaking boundaries is always interesting. For the same reason, episodes where a str8 or timid character is "forced" to break their sexual boundaries, while not changing their orientation are hot. Easy to do in a sissy path, but I'd insist on what I said before - a completely sissy path where the player has no agency to break the rules imposed on them, or exit that path, can be boring as well, if you feel like you're missing out on other aspects of the game... Tricky to reach a balance between agency and enforcement, but there can be ways to make it interesting. If you implement voluntary prostitution, or voluntary jobs that involve some physical contact (like the classic cafe or diner waiter/waitress), there could also be ways to make them imposed on the MC, by blackmail or as punishment? Maybe a dare or wager quest with school mates could be a way to force the MC to experiment beyond their roleplayed boundaries.
Finally, if the engine issue were to be still open, although I've seen someone share their dislike for qsp (which I like), I'd mention the Awesomium version of this. Seems to do exactly what you tried to achieve with html, a fixed layout which is also resizable, hierarchical navigation, menus, pictures and text boxes that can be used to avoid scrolling. Looks solid from the only game I've seen that uses it, World between Worlds. Also, while respecting others' opinion, I've fiddled a little bit with qsp and, being no coder myself, been able to add a few hacks to the game, because it's so easy to edit; the games I know that update more frequently are, by far, the qsp games, some with multiple updates in the same day, which I assume is possible because by design. It's a great way to engage a community around a game, especially while it's still an early WIP.
Just some food for thought, thanks for making this game and wish you luck with it, has a lot of potential.