One possible storyline could be a battle of wits between Leonardo and Tigerclaw. Leonardo, being the leader of the Turtles, would constantly try to outsmart Tigerclaw. For example, he could set up elaborate traps to catch Tigerclaw and stop his evil plans. Tigerclaw, on the other hand, would be constantly trying to anticipate Leonardo's moves and turn them against him. Another storyline could be a more complex relationship where they start to understand each other's motives over time. Maybe there could be a situation where they have to team up against a common, even greater enemy. This could lead to a begrudging respect between them.
One idea could be a story where Leonardo and Tigerclaw are forced to team up temporarily to defeat a common enemy greater than their rivalry. Maybe a new, powerful mutant emerges that threatens both the turtles and Shredder's plans. This would put them in a situation where they have to rely on each other's skills.
The conflict between good and evil is a key element. Leonardo represents the good, and Tigerclaw is clearly on the side of evil. Their clashes, both physical and ideological, can create a lot of tension and excitement. For instance, their different fighting styles - Leonardo's ninjutsu training with Splinter versus Tigerclaw's more brute - force and underhanded tactics. Another element is the mystery around Tigerclaw. There could be parts of his past that are slowly revealed in the fanfiction, which could add depth to the story.
Leonardo would be a likely candidate. He is often the most level - headed of the turtles and believes in honor and justice. He might see the potential for a greater good if he can end the hostilities with Tigerclaw, so he could be the one to make the first move towards a truce.
It often explores themes of family in a new way. Since she's their sister, it's not just about friendship or alliance. There's a sense of loyalty and protection that comes with the family bond. Also, the human sister can bring a different perspective to the turtles' battles and adventures, adding more depth to the story.
The unique part is often the blend of two different worlds. The turtles' ninja and mutant nature against the backdrop of a normal human high school. It's like seeing aliens in a regular school.
One great 'tmnt human sister fanfiction' could be 'Sisterly Bonds'. It focuses on the unique relationship between the human sister and the turtles, with a lot of heartwarming moments as they face various challenges together in New York City.
To write a compelling Leonardo in fanfiction, first focus on his existing traits. He's brave, so show him facing his fears head - on in new and unexpected situations. For example, if he has a fear of heights, put him in a situation where he has to climb a tall building to save someone. Also, explore his relationships. Maybe he has a falling - out with a close friend and has to work hard to mend it, which shows his loyalty and determination.
The movie claims to be rooted in truth, but as with many such accounts, it's likely a blend of facts and artistic license to make for a compelling narrative.