Typically, tanjirou x zenitsu fanfic often focuses on their strong friendship. It might show how Tanjirou's calmness balances Zenitsu's more excitable nature. Also, their shared experiences in the demon - slaying world are a big part of these fics. For example, they might face a powerful demon together and their bond is strengthened through the ordeal.
One popular 'zenitsu x nezuko fanfic' is 'The Unlikely Bond'. It beautifully portrays the relationship between Zenitsu and Nezuko. The author does a great job of showing Zenitsu's protective nature towards Nezuko and how Nezuko's presence calms Zenitsu in his often-fearful state.
Fanfiction.net is also a great place to search for such fanfictions. There are many authors who post their works there. However, the quality may vary, so you might need to read the summaries and reviews to find the really good ones. Some Tumblr blogs also reblog or create 'tanjirou x zenitsu' fanfictions, so it's worth checking out Tumblr as well.
One great 'tanjirou x zenitsu fanfiction' story could be about their adventures together outside of the main plot. Maybe they encounter a new type of demon and have to rely on each other's strengths. Tanjirou's determination and Zenitsu's unique skills would combine in interesting ways to defeat the demon and strengthen their bond.
One common theme is the development of their relationship. It often shows how Zenitsu's initial fear or admiration for Nezuko turns into deeper feelings. Another theme could be their adventures together, fighting demons side by side in some fanfictions.