Perhaps a new villain emerges. A powerful sorcerer who is angry that the turtles have been turned human by a magic that he believes should be under his control. So he tries to either turn them back into turtles in a more twisted way or simply destroy them in their human forms because he sees them as abominations.
One possible plot could be that the turtles are exposed to a strange mutagenic substance that starts to reverse their mutation and gradually turn them human. They then have to deal with the challenges of living as humans, like going to school and fitting into society while also trying to hide their former turtle identities from those who might not understand.
Leonardo is often popular. In human fanfiction, he might be depicted as a strong and noble leader even in human form, guiding the other turtles and any human allies. He could be the one who tries to uphold justice in a human world.
April O'Neil is a classic choice. She's a human character who has a long - standing relationship with the turtles. Her skills as a reporter can add an interesting dynamic to the story.
Krang from Dimension X could also be a great villain. He might be interested in the human sister because she has some unique knowledge or ability that he wants to use for his own evil plans in taking over Earth. He could send his minions to capture her and the Turtles would have to journey to Dimension X to save her.
One idea could be a story where the turtles become human for a day and have to navigate high school. They'd struggle with normal teenage things like fitting in and dealing with bullies, all while trying to hide their turtle identities.
One idea could be a story where the Teenage Mutant Ninja Turtles discover a hidden mecha technology in the sewers. They have to figure out how to use it to defeat a new, powerful enemy. Maybe it gives them new abilities like enhanced speed or the power to fly.
The main characters are the four Teenage Mutant Ninja Turtles: Leonardo, Donatello, Raphael, and Michelangelo. And of course, Splinter and April O'Neil.
Leonardo is often very popular. He's the leader, so fanfictions might focus on his leadership challenges and how he makes decisions for the team.
Casey Jones. He's often overshadowed by the turtles. But in fanfiction, his brash and unpredictable nature could be used to create some really interesting storylines. He could have his own solo adventures that tie into the turtles' world in unexpected ways.
Teenage Mutant Ninja Turtles is an American comic book series created by Kevin Eastman and Peter Laird.