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 in this fanfiction, Leo as a girl discovers a new form of ninjutsu that is more attuned to her female energy. This new power could help the Turtles in a big way, but she also has to train hard to master it. Along the way, she has to prove to herself and her brothers that being a girl doesn't mean she's any less of a ninja.
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.
A great plot could be about Leo and Raph competing in a tournament that's not just about physical strength but also about strategy and wisdom. They start off as rivals in the tournament but soon realize that they need to team up to defeat the real threat, which is a group of villains who are cheating in the tournament. This would show their growth from competitors to comrades. Also, a story where they are separated from their other brothers and have to survive in a harsh environment on their own, relying only on each other's company and skills.
Another interesting plot could be that the reader is a time - traveler from the future. They come back to the present day and meet Leo. Leo is initially suspicious but as they work together to prevent a future disaster that the reader knows about, they fall in love. It adds an element of mystery and adventure to the story. Plus, the time - travel aspect can bring in different versions of the TMNT universe and how they interact with the current situation.
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 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 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.
Her appearance could be developed in a unique way. She might have long hair that she has to figure out how to keep out of the way during battles. Also, her fighting style could be more fluid and graceful compared to the traditional male - dominated style.
Perhaps the mutagen that created the Turtles has some strange reaction when exposed to the Dragon Balls' energy. This could lead to new powers for the Turtles and a whole adventure about controlling and understanding these powers while also dealing with the Dragon Ball Z characters' reactions to them. For example, Vegeta might be initially suspicious and see the Turtles as a threat to his power status.
One idea could be a Winged OC who is from a different dimension and has the power to heal. The OC could encounter the TMNT during a battle where one of them is seriously injured. Another plot could be that the Winged OC is on a quest to find a lost artifact, and the TMNT get involved either by accident or because they also have an interest in the artifact. Or perhaps the Winged OC is a runaway from an evil organization that the TMNT are trying to take down, and they form an alliance.
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.