One idea could be that the Turtles from 1987 end up in the 2003 universe and have to adapt to the darker and more serious tone. They could team up with the 2003 Turtles to fight a common enemy like the Shredder. The 1987 Turtles' more light - hearted and comical ways might initially clash with the 2003 Turtles' brooding nature, but they eventually learn from each other.
A possible plot could involve a new mutagen that is created in the 1987 universe. However, it has unexpected and dangerous side effects. The 2003 Turtles are brought in to help study and stop the mutagen. Along the way, they compare their knowledge of mutagen and find that the 1987 Turtles have some unique insights due to their different experiences with mutants. They combine their skills to destroy the mutagen and prevent a mutant apocalypse.
Maybe there's a time - traveling device that gets the two sets of Turtles mixed up. The 2003 Turtles find themselves in the wacky world of the 1987 series. They have to deal with the less - than - threatening villains at first, but then they discover that a more powerful foe from their own universe has also followed them. They then need to work with the 1987 Turtles to send the enemy back and get themselves home.
One idea could be that the Turtles from 2003 get transported to the 2012 universe and they have to deal with the different versions of their enemies. For example, the Shredder in the 2012 version might have some new powers or tactics that the 2003 Turtles aren't used to.
A possible plot is that an ancient evil that was sealed away in the 2003 universe is accidentally awakened by the arrival of the 2012 turtles. The 2003 turtles initially blame the 2012 ones but then realize they need to work together. They combine their strengths, with the 2012 turtles' modern thinking and the 2003 turtles' deep knowledge of ancient ninja lore, to defeat the evil.
One idea could be that the Turtles end up in the Underground world of Undertale. They might have to navigate through the various regions, like Snowdin or Hotland, using their ninja skills. For example, they could use their stealth to avoid some of the more dangerous monsters. Another plot could involve the characters from Undertale being brought to the Turtles' New York. They would then have to deal with the Foot Clan and other threats in this new, strange environment.
One idea could be that Danny's ghost powers start to malfunction in the TMNT's dimension, causing strange ghostly events that the turtles have to help him fix. Another could be a villain from Danny's world teams up with one of the TMNT's foes, forcing the two groups to work together. Maybe Danny accidentally gets sent to the sewers where the turtles live and they initially mistake him for a new type of mutant.
One idea could be that the turtles from 2012 somehow get transported to the 2018 universe. They could be really confused by the different art styles and the new gadgets the 2018 turtles have. The 2018 turtles, on the other hand, would be intrigued by the more traditional look of the 2012 turtles. This could lead to some funny moments as they try to communicate and figure out how to get the 2012 turtles back to their own universe.
A possible plot for tmnt baby sister fanfiction is that the baby sister finds an ancient artifact that has a connection to the turtles' heritage. This artifact might give her some new abilities or put her in danger as various forces try to take it from her. Maybe she befriends a stray animal that turns out to have some kind of magical or otherworldly connection. The turtles are initially skeptical but then realize that the animal is actually helpful. Additionally, the baby sister could get involved in a school project about mutants which leads to some unexpected revelations about her brothers and their place in the world.
One idea could be an OC who is a new mutant in the city. They could have a unique power like the ability to control plants. At first, they are scared and confused about their new form and powers, but as the story progresses, they learn to use it for good and join the turtles in their fight against the bad guys.
One idea could be that as a girl, Leo has to face different challenges in leading the team. Maybe she has to deal with more doubts from others at first because she's female. But she overcomes them with her skills and determination.
Perhaps a plot where Leo discovers some new powers or abilities that are related to his pregnancy. It could be something that he has to learn to control quickly. And then there could be a story arc about the team finding a way to ensure a safe delivery for Leo, which might involve seeking out some very rare or magical items.
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.