Zingiber
Really Really Experienced
Zingiber, I'd love to see your variant rules. If you do post something like that, please mention it here so that I see it.
So far, this has sent me delving into the SRDs of Apocalypse World, Dungeon World, and Monsterhearts. Early reflections give me cause to credit you, SwampThing, with doing a good job translating the engine into a solo rule set for running a game in Chyoa, and that it will require some effort to do significantly better. It would be easy to tweak your rules a bit to change the game focus, but more work to do something more ambitious.
Apocalypse World has an interesting mechanic where two characters have mutual relationship/influence power over each other ("Hx", help/interfere), which can go up and down over game sessions. Each player also has a "Sex Move" power where when two characters have sex together, they can gain special insight, powers, or relationship influence over one another. If the relationship score hits +4, it goes back down to +1 and XP is scored. Some rule in that direction might make sense -- XP after concentrating interactions with a particular NPC, or maybe a special move after concentrating interactions.
I might also want a mechanic favoring the protagonist pursuing something other than just scoring, giving them cause to aim at an arc of successes. ("You want X; in order to get X, you have to achieve W, Y, Z first"). Could be a little tedious for solo rules, but maybe there's some way to do it. Perhaps each character template has a desired achievement; when achieved, pick a special move and a new template and desire.
Monsterhearts has an interesting mechanic related to each character's "dark side" -- what brings it out, what difficulties it causes when acting from your dark side, and what circumstance brings the character back. I was musing that maybe a fail where the dice indicate doubles could bring out your dark side, bringing the dark side rules into play. Dark side rules would force certain choices or give a bonus or penalty to others.
Monsterhearts has rules related to gaining "strings" -- influence -- over PCs and NPCs, which can be spent to manipulate others or as an advantage during a contested roll. I might try adding Strings as part of the rule set. A partial success could involve giving a String to an NPC, which they could then use to manipulate you -- so NPCs should have some simple goals or motivations that you could be the instrument of -- embarrassing their rivals, for instance. The full game also describes "Conditions" as persistent disadvantages that you have to do something to overcome; those might be easier to track than Strings for a solo game.
Dungeon World has an interesting mechanic related to character death. When your character is on the brink of death, roll an unmodified challenge. On 10+, you survive, though grievously wounded. On 7-9, Death offers a bargain that you may return to life as long as you follow a promise -- never seeing the sun again, perhaps. For an erotic RPG rule set, a similar mechanic might be useful related to picking up an interesting character disadvantage when you fail something critical -- perhaps you pick up a bad reputation, or an aversion that gives you a penalty in some circumstances.
Based on personal taste, I would probably de-emphasize force related to sex (except for Monsterhearts "dark side" rules), allow male and female protagonists, and add rules for male NPCs and M/M activity. I'd probably want separate stories for a "Monsterhearts" style game and a "sex adventure" style game.
So far, nothing concrete written out.