It's funny because a decade ago, this specific pairing would've been a real head-scratcher for most Homestucks. They barely interacted in the comic outside of maybe a couple of frames. But that's exactly the canvas fanfiction loves.
Rivalry is usually the engine. Writers spin it out of their canon roles: Terezi as the blind, justice-obsessed Seer of Mind, playing detective games with rules and logic; Nepeta as the shy, feral-blooded Rogue of Heart, who operates on instinct and raw emotion. One thinks, one feels. It's a classic opposites-attract setup, but the 'rivalry' isn't usually about being enemies. It's about methodology. A fic might have Terezi trying to 'solve' Nepeta like a mystery, cataloging her tells and motives, while Nepeta finds Terezi's rigid structures frustrating, yet intriguing. The push-pull creates fantastic tension.
The bonding, then, becomes about finding a middle language. Maybe Terezi learns to 'see' through scent and sound the way Nepeta tracks by scent and movement. Maybe Nepeta learns to appreciate the strategy behind Terezi's 'games'. I've seen stories where they start as reluctant allies on a doomed timeline, forced to cooperate, and the survival dependency melts into something softer. The bonding feels earned because it bridges such a vast gap in their natures. It's less about romance and more about two very isolated, intense characters finding an unexpected mirror in each other.
Honestly, some of the best fics use their shared connection to troll culture's weirdness as a glue. They both get the whole quadrant system, the instincts, the alienness of it all. That shared context makes the leap from rivalry to understanding feel more plausible than, say, a human pairing would.