In the manga, Tanjiro and Kanao don't end up getting married. The story focuses more on their battles and character development rather than romantic pairings.
One great tanjiro and kanao fanfiction could be where they go on a journey together to find a rare herb for an ailing comrade. In this story, their relationship deepens as they face various obstacles in the forest.
First, study their characters well. Know Tanjiro's kind and determined nature, and Kanao's shy yet strong side. Then, think of a unique plot, like them teaming up to save a group of children from a new type of demon. Start with a strong opening that grabs the reader's attention.
If RWBY were to react to Spy x Family fanfiction, they would bring their own perspectives. Ruby, with her innocent and curious nature, would likely be intrigued by Anya's telepathic abilities. Weiss, being more refined, might analyze the political and social aspects of the Spy x Family world. Blake, having experienced secrecy herself, would empathize with the characters hiding their true identities. Yang would enjoy the comical moments and the action - packed sequences, like when Loid does his spy stunts.
If the My Hero Academia characters were to react to Spy x Family fanfiction, it would be quite a spectacle. Momo Yaoyorozu could be impressed by the creative plots in the fanfiction. She might admire how the Spy x Family world is built with its own set of rules and characters. Kirishima would likely be excited by the action scenes. He'd be all about the times when Loid has to fight or use his spy skills. And Mineta? Well, he might have some inappropriate thoughts, but we'll just ignore that and focus on the more positive reactions of the other characters.