I'm too tired to read everyone's thoughts on the topic, but here's my two cents anyways.
In a lot of games, Romance/Relationships tend to work with some sort of merit system. Do someone a favour or respond with the "right" answer in a conversation and you'll earn points. Enough points and you'll pass the threshold which unlocks, well, romance. As easy as that would be, it's not exactly how it works in real life.
When you create a player character, especially in an RPG, it's a blank slate which you fill out yourself with a backstory, certain characteristics, aspirations and interests. Having your character be an empty canvas is fantastic, because it allows you to place yourself in your characters shoes as it were to enhance your overall gameplay experience.
I'm fairly certain that Anime culture invented a concept where a "blank slate", if you will, allows the viewer/reader/player to "become" the main character, and panders to them in a way where the entire universe you're immersing yourself in is catering to you. A good example are Japanese dating sims. The player character is usually incredibly vague and non-descriptive, which makes people mentally fill out their character's personality with their own. In a story like that, a good example being Katawa Shoujo (Which is fantastic, by the way), every single girl, without fail, seems to fall for this guy who has literally no characteristics of his own, but has a persona projected upon them by the person playing.
I don't think that's the way to go. Skyrim made the same mistake of making every NPC that is marriageable also interested in you, after certain prerequisites. Some people are out of your reach. Some people just think you're a weirdo, a slob or a madman. This means that every NPC would be different. And since the flexibility of the world (presumably) allows players to be so incredibly different in their approach, I think a solid romance system is not in the cards for Sui Generis. Besides that, I don't think there's a lot of obvious practicalities to it, but perhaps that's just me.
Also, if it's romance you're after, I'm available...