I think often our backgrounds, professions and basic orientation(sexual,personality, otherwise) influence our perspective and analysis of stuff like this. I am introverted, cis male, hetero and evenly balanced between right and left brain thinking(creative/analytical). I also have a creative writing degree and a law degree, though I started out in undergrad as a physics major and I later studied multimedia in between undergrad and law school. In addition, I started out playing wargames around 11 or 12 years old, then got into tabletop RPGs and ultimately have been an avid e-gamer(console and PC) for the past 30 years or so. I am middle-aged and American and have lived in a "broken home"(divorce) that became more cohesive as I got older. I have an incest fetish and a mind control fetish, and these informed my fantasies from fairly early on. I think I got the mind control fetish because I was very shy and couldn't conceive of women being attracted to me voluntarily. I have fortunately since discovered this not to be the case!
So, when I look at this very unique and particular game genre--whether you call them "porn games", "h games", "eroge", "adult games" or "fap games"(the latter is my favorite)--I do tend to look first at narrative elements, both because of my writing background and because the narrative sets up the scenario and done well makes it "hot". Second, I look at the gameplay elements--how do you "play" the game, how do you "win", progress, etc. Third, I look at the media elements--the art, music, any voice acting or sound effects, and so forth.
What is the objective of a fap game? To titillate, arouse and entertain, presumably. Secondarily, often, to tell a story that accomplishes all of the above. Hot, interesting, fun. So what makes a story hot, interesting and fun?
It's a given that there will be sexual content in a fap game. Often a lot of fetishes will be featured in a fap game, to make it "hot" for as many players as possible. The art, animation and even sound elements will play a big role in the sex appeal of the game. But what storytelling elements make a game hot, interesting and fun? Some thoughts:
1. The story should be inherently interesting--if it's a suburban household family, what is happening that is interesting, besides the sex? Was there a recent major change in the household? Is there something unusual about someone in the household? Etc.
2. The story should contain some original elements--if you are producing the 500th game set in the aforementioned suburban household, what's different about this one? What's unique about this family or this protagonist?
3. The story should be cohesive with the game play--if there are choices the player makes, the story should reflect the impact of those choices in some way, even if only minor.
4. Humor should add to the story, not distract from it.
5. If the main character is an amoral dirtbag, some explanation for why would be helpful.
6. The actual "visible writing"(narrative and dialogue visible on screen) should be as well written as possible, and with the intent to evoke an emotional response from the player as appropriate(e.g., when dialogue happens during sex, it should seem "hot" to the player).
7. Character decisions in-game should seem at least remotely plausible. If the player doesn't believe a character would agree to something so radically different from her normal course of action/comfort zone, it can take them out of the game.
8. Spend more than a minute establishing the characterization of major game characters. Give a real sense of their personality.
9. Follow through to the end--don't half ass the game ending. A game with a "big bang"(lol) at the end is more memorable than one that just kinda fades out after the last conquest is complete.
10. Make the main character more than a blank slate--give them a backstory, personality and some motivational "seeds" to be explored in game.
That's it for now.