The sheer volume of 'Cartman x Kyle' stuff still surprises me sometimes, considering their canonical dynamic is basically pure antagonism. But that's the whole point, right? People love taking that nuclear-level hatred and seeing if there's something else buried under all the insults and schemes. A lot of fics hinge on a sudden, often violent, shift—like one of them getting seriously hurt and the other realizing their feelings aren't just hate. You see a ton of 'enemies to lovers' with a really dark, obsessive edge; Cartman's fixation on Kyle is already there, so twisting it into a possessive, messed-up romance feels like a short hop. Angst is huge, usually stemming from the impossibility of it all within South Park's universe and their friend group. Also, high school AUs where they're not cartoon kids but teenagers, which lets writers explore the rivalry-tension in a more traditionally shippable setting.
Honestly, the most compelling ones for me are the ones that don't soften Cartman too much. If he's just a secretly sweet tsundere, it rings false. The better stories keep his manipulative, selfish core but have that focused entirely on Kyle, which creates a terrifying yet fascinating dynamic. The themes circle guilt, obsession, and the blurry line between hate and a very twisted kind of love. Less popular but still around are lighter, crack-treated-seriously fics that play the whole thing for absurd humor, which honestly feels very true to the source material's spirit.