Well, 'Ao Haru Ride' is a great anime and the idea of a Kou x Futaba fanfic can be really interesting. A good fanfic in this pairing might explore their relationship further. It could delve into their feelings that were not fully shown in the anime, like the small moments of hesitation or the unspoken words between them.
Mostly, it's not appropriate. 'Futaba' is often a character from some works, and creating a 'pregnant' fanfic about a character can be seen as objectifying the character and going against the original intention of the work. Also, it may make some fans who like the original work feel uncomfortable.
You can start by looking on fanfiction.net. It has a large collection of 'Ao Haru Ride' fanfics, including those focused on Kou x Futaba. Another option is Archive of Our Own (AO3). It has a great tagging system which makes it easier to find specific pairings like this one.
The jealousy can be resolved through communication. If Kou is jealous, he could simply talk to Futaba about his feelings. For example, he could say 'I saw you with that other guy and it made me feel uneasy.' And Futaba could then explain the situation. The same goes if Futaba is the jealous one.
One great 'Joker x Reader Fanfiction' is 'The Darkest Temptation'. It delves deep into the complex relationship between the Joker and the reader. The author does an amazing job of portraying the Joker's unpredictable nature while also developing a connection with the reader character. Another good one is 'Joker's Obsession'. It shows how the Joker can be both terrifying and oddly alluring in his pursuit of the reader.
In Harley Quinn x Joker fanfic, the character development is often a focus. Harley's transformation from a psychiatrist to a criminal sidekick is interesting. Writers may play with different timelines and scenarios in the fanfic. For example, they might explore what would happen if Harley finally broke free from the Joker's influence earlier than in the original story. This can lead to various plot twists and new character arcs for both Harley and the Joker.
Technology and hacking are also quite common themes in futaba sakura fanfics. Since she is known for her skills in this area, many fanfics will have her using her hacking abilities to solve problems or get into exciting adventures. Also, her relationship with her past traumas and how she deals with them is often explored in fanfiction.
Good akira and futaba fanfic often has well - developed characters. It should stay true to their personalities as in the original work. For example, Futaba's intelligence and social awkwardness, and Akira's leadership qualities.
I'm not sure if there is a specific 'futaba sakura belly ache fanfic' readily available. You could try searching on some fanfic platforms like Archive of Our Own or FanFiction.net.