In many 'TMNT Mikey hurt' fanfictions, a common plot element is Mikey's own attitude towards his injury. Sometimes he tries to be brave and not show how much pain he's in to not worry his brothers. Another element is the search for a cure or solution. If he's poisoned, they'll look for an antidote. If it's a physical wound, they'll try to find medical supplies or a way to heal it. And there's usually a lot of brotherly bonding as they take care of Mikey.
One possible scenario could be that Mikey gets hurt during a fight with the Foot Clan. Maybe he underestimated an opponent and got a nasty cut on his arm. His brothers would then rush to his side, with Donnie trying to patch him up as quickly as possible while Leo and Raph stand guard, ready to fend off any further attacks.
In a fanfiction, Mikey might get hurt during a mission to infiltrate a Kraang base. He could be detected by security systems and get zapped by some sort of electric defense mechanism. This would leave him weak and in pain. His brothers would need to carry him out of there and find a way to reverse the effects of the zap. They'd be desperate to get Mikey back to his normal, energetic self.
One common plot element could be the other turtles rallying around Mikey. They might try to find a cure or take care of him. For example, Donnie could use his scientific knowledge to diagnose Mikey. Another element could be Mikey's own struggle with being sick. He might feel weak and frustrated that he can't do his normal activities. Also, there could be a threat that takes advantage of Mikey's weakened state, like an enemy planning an attack while he's down.
In a fanfiction, Mikey might get hurt while trying to protect his brothers. For example, if the Kraang were attacking and his brothers were in danger, Mikey could jump in front of a powerful blast meant for them. This would leave him seriously injured, with his brothers then having to take care of him and find a way to heal him.
A great 'TMNT Mikey hurt' fanfiction is 'Mikey's Painful Encounter'. In this story, Mikey is hurt by a mutagen - related accident. His brothers are desperate to find a cure, and the fic does a good job of showing their emotional turmoil. There are also some really interesting side characters in this fanfic that add depth to the story. Additionally, 'Hurt Mikey, Stronger Spirit' is a fanfic where Mikey endures a physical injury but his spirit remains unbroken, and it's inspiring to read how he overcomes his pain.
Transformation is also a key theme. Dark Mikey has obviously transformed into a darker version of himself, and Karai may also experience some sort of change in relation to him. This can be physical, like in a fight where they both gain new skills or powers, or it can be emotional, such as growing to trust or distrust each other more. Additionally, the theme of mystery often comes into play. Karai has always been a bit of an enigma, and Dark Mikey's new state adds another layer of mystery to their relationship.
Another theme might be adventure. Given their nature as turtles who often go on adventures, the fanfiction might incorporate their adventures in a way that also showcases their relationship. For example, they could be on a mission together and their bond is tested or strengthened during that time. It could also involve themes of self - discovery, as they learn more about themselves and each other through their interactions.
One common theme could be reconciliation. Maybe they've had a huge fight and the kiss is a way to make up. Another could be a moment of shared joy or celebration. It might also be used as a way to show a new level of understanding between them.
It could be that they were tricked into thinking they were helping someone. A villain might have convinced them that the 'kidnapped' person was a bad guy who needed to be taken away for interrogation. But as the story unfolds, they realize they've been used and have to find a way to make things right.
One idea could be that Mikey and Shinigami team up to solve a mystery in the sewers that involves some strange magical creatures. They use Mikey's fun - loving nature and Shinigami's magic skills to figure things out.