the sheer variety is wild. The most common pairing is obviously Lan Wangji/Wei Wuxian, but the way people approach it splits into distinct lanes. There's a massive amount of post-canon domestic fluff—stuff about them raising A-Yuan, bickering over Gusu rules, that kind of warm blanket comfort. Then you've got the complete opposite: intense canon-divergence where Wei Wuxian never died, or where he returns much earlier. Those often get into political intrigue the novel only hinted at.
A trope I see constantly and will never get tired of is 'soulmate-identifying marks' AUs. It's like the fandom's favorite sandbox. Sometimes Lan Zhan has Wei Ying's name etched on his skin, sometimes they share a red thread, sometimes they dream each other's memories. It just fits the whole destined, fated romance vibe of the original so well, even if it's a modern setting. The other huge one is time travel fix-its, usually with Wei Wuxian going back to his teenage years armed with future knowledge. They're basically power fantasies, but the good ones really dig into the regret and the second chances.
Less popular but still thriving are the rare pairs. Nie Mingjue/Jin Guangyao has a dedicated, slightly masochistic following for all the angst potential. Jiang Cheng-centric gen fics exploring his leadership or his relationships post-canon are often really well-written. And you can't forget the Xue Yang/Xiao Xingchen dynamic—it's a tragedy magnet, but people keep writing it, maybe to somehow heal that unbearable ending.