One interesting story could be about the little sister getting involved in the turtles' training sessions and bringing a new dynamic to their routine. For example, she might show them a different way of approaching a combat move.
One popular one could be 'The Turtles and Their Little Sister's Adventure'. It often focuses on how the turtles protect their little sister in the dangerous world they live in. They face off against their usual enemies like the Foot Clan while also teaching their sister important skills.
The little sister could also be a key in uniting different factions in the TMNT universe. For example, she might befriend some of the Foot Clan members who are tired of Shredder's tyranny. Her innocent nature could make them see a different side of life. And with the help of her turtle brothers, they could work together to bring peace to the city. It would be a great plot twist and a way to explore new relationships in the TMNT world.
She could be the moral compass for the turtles. Since she's younger, she might see things in a more innocent way and remind the turtles about the importance of things like mercy and forgiveness. For example, when they face an enemy, she could persuade them to give the enemy a second chance.
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 that the little human sister discovers the Tmnt's lair by accident. Maybe she was exploring the sewers for a school project on urban infrastructure. Once there, she befriends the turtles and helps them out with some human - world tasks like getting supplies or finding information on their latest enemy.
Some fanfictions might introduce new characters as friends or allies of the little sister. For example, there could be a street - smart kid who helps the little sister understand the human world better since she's growing up in a rather unusual family. But the main focus always remains on the relationship between the turtles and their little sister.
One possible story could be that the TMNT find their little sister feeling down one day. To cheer her up, they start a gentle tickling session. The little sister initially protests but soon erupts into laughter. It becomes a fun bonding moment for them all.
A great fanfiction idea could be a Little Sister's rebellion against the established order in Rapture. She starts to question why she has to harvest ADAM and decides to fight against those who control her. This could involve her teaming up with other splicers who also want to change the system in Rapture.
There could be a fanfiction where Naruto's little sister is sent on a special mission. She has to go undercover in an enemy village. Along the way, she makes new friends who are initially suspicious of her but come to respect her as they see her determination and kind heart. She also uncovers a plot that could endanger both her village and the one she's infiltrated.
Some 'hetalia little sister fanfiction' focuses on her daily life. She could be living with one of the Hetalia characters and having to deal with their strange habits. For instance, if she lives with Italy, she might have to get used to his love for pasta and his laid - back attitude towards everything.