One common theme is adventure. Since both Kenichi and the world of Katanagatari have elements of action and exploration, fanfics often bring these two together in exciting adventures. Another theme could be self - discovery. The characters may learn more about themselves as they interact with each other from different universes.
Well, I haven't read many 'kenichi x katanagatari fanfics' myself. However, you could also check out Archive of Our Own (AO3). It has a large collection of fan - created works. You might have to dig a bit through the search results, but chances are you'll find some fanfics that suit your taste regarding the kenichi x katanagatari pairing.
One common theme is adventure. Writers often put them in exciting situations where they have to face challenges together, like exploring a mysterious land or fighting against a powerful enemy.
A major theme is also the exploration of different cultures within the two fictional universes. The ninja world of Naruto and the martial arts world of Kenichi have their own unique cultures. Fanfic writers often use this to create interesting scenarios. For instance, they might have Kenichi participate in a ninja festival in the Naruto world, or Naruto train in a traditional martial arts dojo in Kenichi's world. This theme of cultural exchange adds depth to the stories.
Another great one is 'Kenichi and Renka: A New Beginning'. It starts with a fresh take on their relationship. Maybe they meet in a different context than in the original story. It then goes on to show how they interact, get to know each other better, and gradually form a special connection.
One common theme could be the growth of their relationship. It might show how they start from being just friends or comrades and gradually develop deeper feelings for each other.
There might be a 'kenichi x kisara fanfic' that focuses on their relationship development. For example, it could start with them as friends and gradually show how they fall in love through shared experiences like helping each other with personal problems or working towards a common goal in their school or community.
Some dedicated anime or manga fan communities might also be a good source. These communities often have their own sections for fan - created works, including fanfics about characters like Kenichi and Kisara. You can join these communities and ask around for 'kenichi x kisara fanfic' recommendations.
Another option could be FanFiction.net. It's been around for a long time and has a wide range of fan - created content. Just use the search function and look for kenichi x shigure. However, the quality may vary, so you might need to do some sifting through the results.