I have a few:
- Chosen ones, fate, destiny, &c. When you get down to it, a story with these themes is one where a single person or handful of people is ontologically, cosmically better and more important than everyone else. It's eerily similar to that right-wing meme about how "most people are just NPCs" (though I disliked the trope before that meme ever took off).
- Way too much importance being given to bloodlines by the narrative (note, this is different from them being given importance by characters or societies in the story).
- All of the good characters are handsome and beautiful, while all of the evil characters are ugly and disfigured (with the possible exception of a femme fatale or two).
- Races that are inherently, unchangeably evil down to the last individual regardless of upbringing, society, or material circumstances.
You're absolutely right. I wouldn't mind seeing it explored in fiction and fantasy in particular, which seems to me like the perfect genre to explore those dynamics. The only other story I can think that does it also features a sort of throuple, though the two women have a more sisterly relationship while both are in a romantic relationship only with the male (the Honorverse series)
I hope with queerness and non-heteronormativity becoming more "mainstream" (well, talked about more openly outside of niche spaces) that we'll start to see more of an exploration of what being poly means