I've noticed a few patterns that pop up constantly with these two. The classic enemies-to-lovers framework is huge, obviously, because their official dynamic has that built-in competitive edge. But the way writers twist it is what's interesting. A lot of fics start with them as genuine rivals—maybe over a gambling debt, a festival planning committee gone wrong, or Ayato deliberately provoking Itto to test his limits—and then the tension just... melts into something else. The 'forced cohabitation' trope is another massive one. A storm strands them, a youkai curse binds them together, the Shuumatsuban needs Itto under close surveillance—any excuse to shove these two polar opposites into a shared space and watch the chaos and eventual softness unfold.
There's also a surprising amount of fics that lean into the 'caretaker' trope, but it's rarely one-sided. Sometimes Itto gets injured protecting someone and Ayato, feeling responsible or just oddly compelled, has to deal with this boisterous oni recovering in his estate, being a terrible patient. Other times, it's Ayato who collapses from overwork, and Itto, who accidentally discovers him, becomes this unexpectedly gentle, stubborn presence ensuring he actually rests. I think the appeal there is flipping expectations: the refined commissioner being cared for by the 'uncouth' oni, and the oni revealing a protective, attentive side he usually hides behind bravado. You also see a lot of 'fake dating' for political or festival purposes, which is just a perfect vehicle for both comedy and eventual real feelings. The trope of Ayato secretly admiring Itto's uncomplicated freedom, or Itto seeing through Ayato's polite masks, is the emotional core of like, 70% of these stories regardless of plot device.