Yes, there are quite a few. You can start by looking on popular fanfiction websites. Some might focus on their relationship dynamics in the context of the original story's world, exploring how their different personalities interact.
Another option is Tumblr. There are often fanfic writers who post their works there, and you might be able to find some Himeno x Denji fanfiction by searching relevant tags or following the right blogs.
One common theme is the exploration of their relationship development. It could start from their initial meeting and gradually show how they grow closer.
Another characteristic is the exploration of their relationship. It can show different aspects of their bond, whether it's friendship, a deeper emotional connection, or something in between. The fanfic should also be in line with the general 'feel' of the original source material, if there is any, in terms of the setting, mood, and the nature of the characters.
Another theme is growth. As the story progresses, both Shima and Aki may grow as characters. For example, they could learn important life lessons from their interactions with each other. In some fanfictions, they overcome personal weaknesses with the help of their relationship.
Another theme could be self - discovery. The characters might be on a journey to discover more about themselves, and their relationship helps in this process. For example, Aki might learn something new about herself through her interactions with Grey, and vice versa.
Well, 'The Bond between Aki and Shima' is a great aki x shima fanfic too. It focuses on how their bond develops over time. The author vividly describes their interactions, from the first shy glances to the deep conversations, showing a beautiful relationship.
Fanfiction.net is also a great place to look for masamune x aki fanfiction. There are many writers who contribute to this pairing. However, the quality may vary, so you might need to read through a few to find the really good ones. You can also check out some dedicated anime or manga fanfiction forums where fans might share their works or recommend some good ones.