Kuroko Tetsuya is definitely a popular choice. His unique 'phantom' playing style can create a lot of interesting scenarios in the Extra Game. Writers can explore his growth further and how he uses his skills against new opponents.
A plot could revolve around a mysterious new player who suddenly appears in the Extra Game. This player has extraordinary skills but a very secretive past. The story could be about the main characters trying to figure out who this player is and what their motives are while also competing against them on the court.
You can try some popular anime streaming platforms like Crunchyroll or Funimation. Sometimes, they might have related fan-made content. However, make sure it complies with their terms of use.
One highly recommended fanfiction story is 'The Miraculous Return'. It has an interesting plot that continues the story of the original anime in a creative way.
Another theme is the exploration of their individual insecurities. In fanfiction, they might face their own weaknesses and how they overcome them with the help of each other. For example, Aomine's slump and Kagami's struggle to reach the top level can be a part of the story. Also, their shared passion for basketball is always a central theme. It's what brought them together in the first place and continues to be a major factor in their relationship in fanfiction.
One great 'Kuroko no Basuke Aomine x Kagami' fanfiction is 'Rivals to Friends'. It explores their relationship in a really interesting way, showing how their rivalry on the court can transform into a deeper understanding and friendship off the court.
I really like 'Kuroko's New Adventure'. In this fanfic, Kuroko discovers a new basketball technique that challenges him and his team. It's full of action - packed basketball scenes and also delves into the relationships between the characters, adding more depth to their bonds.
His friends from his original life could be main characters. For example, if he has a best friend, that person would play an important role in helping Kuroko adjust to being a girl. Maybe they would offer support and advice, or just be there to listen when Kuroko has a tough time dealing with the changes.
One popular Yuri on Ice and Haikyuu crossover fanfiction is 'On Ice Courts'. It combines the figure skating elements from Yuri on Ice with the volleyball action of Haikyuu in a really interesting way. The characters from both shows interact in a fictional sports event that brings out new aspects of their personalities.
Many kise x kuroko fanfic explore their friendship or a deeper relationship. They might show how they support each other in the basketball court and in their personal lives. For example, in some fanfics, Kise helps Kuroko come out of his shell more, and Kuroko helps Kise to be more in tune with his emotions. These stories usually have a strong emotional core that revolves around their bond.