A plotline could be that Haruki gets into trouble and Isuke has to save her. This event could be the catalyst for their relationship to develop. Isuke, being brave and resourceful, manages to rescue Haruki from a dangerous situation, and in the aftermath, they start to see each other in a new light. It could also be that they are both outsiders in their school or group in the Akuma no Riddle setting, and they find solace in each other's company, leading to a romantic relationship.
Isuke could start as a very guarded character, but through interactions with Haruki, become more open and trusting. Haruki, on the other hand, might become more patient and understanding. For instance, if Isuke has trust issues due to past experiences, Haruki's kindness could slowly break down those walls, and Isuke would learn to be more vulnerable.
One possible plot could be that Isuke and Haruki start off as rivals in the dangerous world of Akuma no Riddle. They might be competing for a certain goal or mission. As they interact more, they gradually discover they have more in common than they thought. Their relationship then evolves from rivalry to friendship and perhaps something more romantic as they face various challenges together.
Their relationship could be full of tension at first. Isuke might be more reserved and Haruki more outgoing, creating an interesting dynamic. As the story progresses, they might start to complement each other. For example, Isuke's calmness could balance Haruki's impulsiveness.
One of the main themes is mutual support. In most of these fanfictions, Isuke and Haruki are there for each other through thick and thin. They support each other's dreams and goals. Also, the theme of discovery is quite common. They discover new things about themselves and about each other as the story progresses. It could be a new hobby or a hidden talent. And of course, the theme of love, whether it's platonic or romantic, is always there in some form.
One great story could be 'The Unlikely Bond' where Haruki Sagae and Isuke start off as rivals but gradually develop a deep friendship that might turn into something more. It has a really engaging plot with great character development.
One popular isuke x haruki fanfiction is 'A Bond Beyond Words'. It beautifully portrays their relationship development. Another great one is 'Shared Moments' which focuses on their adventures together. And 'The Unspoken Connection' also shows a very deep and emotional bond between them.
A possible plot element could be a mystery that Tokaku and Haru have to solve together. This would play on the 'riddle' aspect of the original 'akuma no riddle' concept.
One plot idea could be that Banba and Hanabusa are on a secret mission together. They face various challenges that test their skills and their relationship. Along the way, they start to understand each other on a deeper level and develop a stronger bond.
You can start by searching on popular fanfiction websites like Archive of Our Own. Just type in 'haruki sagae x isuke' in the search bar and you'll get a list of relevant stories.
Since I haven't read this particular 'akuma no riddle banba fanfic', it's hard to say for certain. However, generally in fanfics for 'Akuma no Riddle', they often explore the relationships between the characters in more depth. Maybe this one with 'Banba' focuses on his backstory, or his relationships with other characters in a way that the original work didn't fully explore.