You can start by looking on fanfiction platforms like Archive of Our Own (AO3). It has a large collection of fanfics across various fandoms. Just search for 'Toshiki Kai x Misaki Tokura' and you'll likely find a good number of stories. Another option is FanFiction.net, although the quality can be a bit more variable there.
One common theme is friendship development. Since they are characters from the same universe, many fanfics explore how their friendship could grow deeper over time. Another theme is adventure. Writers might create scenarios where they go on quests or face challenges together.
In Naruto fanfic, Inoue Shiori might be depicted as a very intelligent character. Since the Naruto world values intelligence for strategies and understanding complex jutsus, she could be the one who comes up with clever plans.
I'm not entirely sure what specific 'ichigo x inoue lemon fanfic' you are referring to. But generally, if it's a fanfic about Ichigo and Inoue, it might explore their relationship in a more romantic or emotional way. The 'lemon' aspect could imply some level of more mature or spicy content, though it depends on how the author has written it.
Another theme is friendship. Even if it has a romantic undertone, their friendship is usually a big part of the story. It shows how they support each other through different challenges.
There are also fanfictions that focus on the 'what - if' scenarios. What if they got together earlier? What if they had to face different enemies as a couple? These themes allow the authors to deviate from the original story and create unique and interesting plotlines centered around inoue x ichigo.