This one's fascinating because 'Demon Slayer' fanfic has such a wild variety of AUs, and Yoriichi's tragic, powerful character lends itself to so many 'what if' scenarios. A huge favorite is the modern soulmate AU, where he's this quiet, unnaturally strong guy with a mysterious past, and the reader is the one who can see his Hanafuda earrings glowing or has matching marks. It plays with the idea of his canon loneliness being reversed in a peaceful setting, which is a massive wish-fulfillment draw.
Another super common one is the 'Yoriichi survives' alternate timeline. The reader is a demon he saves and protects, or maybe a demon slayer who finds him centuries later, and the story deals with him navigating a world where Muzan is gone but he's essentially a relic. I've seen a lot of angst mixed with slice-of-life moments, like him learning to cook in a modern kitchen or dealing with technology while keeping his swordsmanship secret.
I also think the 'reincarnation' or 'transmigration' AU is huge, where the reader wakes up in the Sengoku period, maybe as a village healer or even a demon with unusual pacifist tendencies, and their presence subtly changes his path. These stories often focus on quiet emotional intimacy because his canon character isn't overly expressive, so small gestures carry a lot of weight. The appeal is giving that legendary figure a chance at a normal, happy connection, which the original story so brutally denied him.