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.