Hahaha, you asked for a novel.



Well, since they didn't talk much in canon, of course it's sort of informed by the relationships between Kankri and Meenah, and the Condesce and Sufferer.



Personally I really like caliginous ships, and there is potential for resentment/contempt, even though it never manifested, with the royal heiress not ever getting to inherit her throne, while the mutantblood becomes the team leader solely for his people skills, even though he's also really grouchy. Meanwhile, Feferi is so friendly and giving and nice, and no one rewards her for that. It doesn't seem like they ever really hated each other over that, but a shipper can dream. (Also, in a pesterlog between Feferi and Eridan, Fef complains that Eridan has been gossiping with Karkat so much that he has no gossip left over for his own moirail. Is she jealous? A jealousy that could maybe go somewhere?)



On the other hand, if you want to take it flushed, or maybe even pale, Feferi wants to turn around the meaning of "cull," and essentially cuddle lowbloods, and there is none lower than Karkat, and there is none more resistant to cuddling than Karkat. Because he'd make it a chase, she'd get more invested in personally improving his life and making him smile. And he can't keep on saying no forever to the highest blood telling him there's nothing wrong with his blood and that he's safe.