If we're talking about Hualian pairings beyond the main canon, I find myself drawn most to AUs that preserve the core dynamic but shift the context completely. Modern college AUs where Hua Cheng is the campus bad boy and Xie Lian the slightly oblivious, perpetually unlucky scholarship student just hit different. There's something about translating Xie Lian's celestial patience into surviving student loans and bad roommates that makes his eventual devotion from Hua Cheng feel even more grounded.
Another pairing that lives in my head rent-free is the 'Ghost King Hua Cheng & Mortal Cultivator Xie Lian' trope, but set before their reunion. Fics where Hua Cheng secretly watches over Xie Lian's mortal struggles, intervening just enough to keep him from true disaster but never revealing himself, are a special kind of agony. It's all about the pining and the protection from the shadows. I tend to avoid fics that pair either of them with other characters from the TGCF universe romantically—it just feels off to me, given the canon intensity. The best fics for me are the ones that explore different facets of their established bond, not replace it.