The 8th child could play the role of a mediator. Since they are new to the group, they might be more objective and able to see the problems between the siblings more clearly. They could use their unique power, say the power of teleportation, to bring the siblings together when they are at odds.
The 8th child in the Umbrella Academy fanfiction might be a character with a tragic backstory. They could have been separated from the others at a young age and grew up in a different environment. Their power could be something related to nature, like the ability to control plants. When they finally meet the Umbrella Academy, they bring with them a different perspective and a new set of skills that could change the dynamics of the whole group.
In a Luther and Allison fanfiction, there could be a lot of emotional moments. For example, they might have to face their past traumas together. Since they are siblings in the Umbrella Academy, the fanfiction could explore a deeper bond that goes beyond the typical sibling relationship, like they rely on each other more than ever to deal with the chaos of their lives.
Some common themes could be the exploration of the children's powers in unique ways. For example, in fanfic, a writer might focus on how a particular child's power affects their relationships with other academy members. Another theme could be the children's struggle with their identity outside of their superhero personas, as they are still kids dealing with normal growing - up issues.
The human child could be a messenger. They could bring news from the outside world to the Valley of Peace, which might be about a new enemy or a great danger approaching.
One possible plot could be that the Umbrella Academy members are somehow transported to the Naruto universe. They might initially be seen as strange intruders. Five, with his time - traveling abilities, could be intrigued by the ninja time - travel jutsus. Klaus might make friends with some of the more spiritual ninja due to his ability to see the dead. And they could all team up with Naruto and his friends to fight a common enemy, like a new and powerful Akatsuki - like organization that threatens both their worlds.
She could run away to a small town and discover she has a natural talent for music there. She starts performing in local cafes and gradually builds a new life for herself, away from the chaos of the Umbrella Academy.
One theme could be acceptance. Five could accept Vanya fully, powers and all, which would be a great comfort to her. He might tell her that her powers don't make her a monster but rather a unique and important part of the family.
Well, it could be really cool. The Umbrella Academy has its own unique set of characters with superpowers and a rather dysfunctional family dynamic. The Bat Family, on the other hand, is more about detective skills and a darker, grittier world. In a crossover fanfiction, perhaps the Umbrella Academy members could visit Gotham. Five could use his time - traveling abilities to mess with the Bat Family's timelines. Vanya's powerful sound waves might be a new threat that the Bat Family has to face. And the Bat Family's combat skills and detective work could be put to the test against the strange powers of the Umbrella Academy.
One common theme is time - travel. Since the show has elements of time - travel, many fanfictions explore it further. Another is the relationships between the siblings, like their reconciliations or new conflicts.
Character - study fanfiction is a great alternative. It can explore the backstories and inner thoughts of the characters in more depth. For example, you could find fanfiction that focuses on Five's time - traveling experiences and how it has shaped his personality. There are also fanfictions that focus on the group dynamics and how they work together to solve problems in the academy.