One idea could be that the aliens are actually friendly but are misunderstood at first. They came to Earth to learn about human emotions and end up helping the family in the attic deal with their own internal problems.
Perhaps the aliens in the attic have a special power that the kids in the house discover. This power could be something like the ability to communicate with animals, and they all go on an adventure to save the local wildlife reserve from a developer. So the fanfic could be about the cooperation between the kids and the aliens.
The aliens in the attic could be in hiding from a more hostile alien race. The family living in the house unknowingly provides them with shelter. As the story progresses, the family and the aliens form an alliance to fight off the approaching threat.
Well, there could be a brave kid character. This kid is not afraid to explore the attic and is the first to encounter the aliens. He or she then becomes the bridge between the aliens and the rest of the family.
One idea could be an alien oc who has the ability to blend into human society but is on a secret mission to protect Earth from a greater alien threat. Another could be an alien oc with a unique form of communication that causes chaos when humans try to understand it. Maybe an alien oc that is stranded on Earth and has to adapt to human customs in order to survive and find a way back home.
Perhaps a plot where the human - like aliens are being hunted by an evil organization that wants to study and use their powers. They have to rely on their wits and the help of a few trusted humans. Diamondhead as a human could use his crystal - like skin ability in creative ways to defend against the hunters. And they could also start to question whether they are more alien or more human as they fight for their freedom, which would be a great exploration of identity in the context of this fanfic.
One idea could be that the main character, after being adopted by aliens, discovers they have a special power that is crucial for the survival of the alien race. Another could be that the character has to adapt to the strange alien customs and family structure while also trying to find a way back to Earth. Maybe they also form an unexpected friendship with an alien outcast.
One idea could be a story where the monsters and aliens have to team up against an even greater threat from another dimension. Maybe a powerful interdimensional being that wants to conquer both Earth and the alien planets.
An old grandfather who lives in the house could be a main character. He has a lot of stories and wisdom. Maybe he has had some previous encounter with aliens when he was young and now he can help the family deal with the situation in the attic. He could be the one to bridge the gap between the humans and the aliens.
One plot idea could be that aliens invade the Naruto world seeking a powerful energy source, which is actually the chakra. The Naruto monsters then team up to defend their world. Another could be a group of Naruto monsters getting accidentally transported to an alien planet and having to fight their way back home. They could learn new techniques from the aliens they encounter during their journey.
One idea could be exploring the backstories of the minor characters in 'Flowers in the Attic'. For example, what was the life of the grandmother like before the events of the main story? Another could be a different ending where the children manage to escape earlier and build new lives. Or perhaps a fan fiction that focuses on the relationship between the mother and the children from her perspective, explaining her actions more in - depth.
Another plot could involve a scientific discovery about the origin of the Aliens or Predators that has far - reaching implications for the universe. Maybe the discovery could lead to a way to control or even eliminate one of the species, but there are moral dilemmas attached.