In the fanfiction, it might be someone from his own team. Maybe there's a new member who is insecure and feels that Kuroko is getting too much attention. So, in a fit of jealousy, this person does something that causes Kuroko to get hurt, like tampering with his equipment.
One common plotline is Kuroko getting injured during a basketball game. This could be due to a rough play from an opposing team or an accident on the court. Then his teammates would usually be very concerned and try to help him recover.
In 'kuroko gets hurt fanfiction', he could be injured in a fight. For example, if some rivals from another school pick a fight with him because of his basketball skills. Also, perhaps while training alone at night, he overexerts himself and pulls a muscle. It's also possible that in the story, he gets hurt trying to protect one of his teammates from a bully.
You can try searching on fanfiction websites like Archive of Our Own (AO3) or Fanfiction.net. Just type in 'Kuroko hurt' in the search bar, and it should show relevant stories.
In such a fanfiction, perhaps it could start with Kuroko being targeted by a rival team who want to disrupt Seirin's plays. They might kidnap him on his way home from practice. His teammates would then start a frantic search, led by Kagami. They would scour the city, following any leads they could find. In the end, they manage to rescue Kuroko and their bond becomes even stronger through this ordeal.
A possible plot for a Female Kuroko x Akashi fanfiction could be set in an alternate universe where there are no gender - based restrictions in basketball teams. Female Kuroko joins Rakuzan instead of Seirin. Akashi, as the captain, is initially skeptical but soon realizes her potential. They start training together intensively. As they do so, they begin to develop feelings for each other. However, they face opposition from some of the team members who are not used to the idea of their captain being involved with a female teammate. But through their strong bond and love for basketball, they overcome these obstacles and their relationship blossoms.
One great Akashi x Kuroko fanfiction is 'The Red and the Blue'. It beautifully delves into their relationship, showing how Akashi's dominant personality and Kuroko's calm nature complement each other. The author does an excellent job of portraying their emotions in a Yaoi context.
The kidnappers could be motivated by jealousy. They might see Kuroko's skills and his importance to Seirin and want to take him out of the picture so their own team can have an advantage. For example, a rival team that has been losing to Seirin might think that without Kuroko, Seirin will be easier to defeat.
One common theme is the exploration of their complex relationship. Given their different personalities and playing styles on the court, fanfictions often delve into how they understand and support each other off the court. For example, how Akashi's strict and calculating nature might be softened by Kuroko's quiet and observant presence.
A new player on an opposing team could be the cause. This new player might be overly cocky and disrespectful towards Kuroko during a game. They could constantly taunt him about his lack of presence, which finally gets on Kuroko's nerves and makes him mad.