Ultra Beasts can fit into the Pokemon Hall of Origin story in a fanfic by being the guardians of the Hall. Maybe they were placed there by some ancient power to protect the secrets within. So when the protagonist tries to enter the Hall of Origin, they have to face these Ultra Beasts.
They could also be the result of an experiment that went wrong in the Hall of Origin. The Hall's powerful energy could have accidentally opened portals to the Ultra Beast's dimension long ago. In the fanfic, the main character could be on a journey to reverse this accident and close the portals by exploring the Hall of Origin and learning about the Ultra Beasts.
Well, in a fanfic about the Pokemon Hall of Origin and Ultra Beasts, you can expect a wild adventure. The Hall of Origin is a very mysterious place in the Pokemon world. Ultra Beasts are strange and powerful creatures from another dimension. A fanfic might explore how a trainer gets involved with the Ultra Beasts near the Hall of Origin. Maybe they accidentally open a portal to the Ultra Beast's world while exploring the Hall. It could also show battles between the trainer's Pokemon and the Ultra Beasts, with unique strategies based on the Ultra Beasts' strange abilities.
Create unique physical traits. Maybe an Ultra Beast has a hidden ability that is only visible when it's in a certain mood, like glowing patterns on its body. Also, you could give it a special way of moving, like floating in a spiral pattern instead of walking.
Well, it could be about a trainer who discovers a hidden entrance to the Hall of Origin. There, they meet legendary Pokemon and learn ancient secrets of Pokemon creation.
It could be about a human who discovers a hidden entrance to the Pokemon Hall of Origin. This human might have a special bond with a particular Pokemon that leads them there. Maybe they are on a quest to learn the secrets of Pokemon creation or to find a way to stop an evil force that threatens the balance in the Pokemon world.
It might be that the fanfic focuses on a trainer who discovers Necrozma's link to the Hall of Origin. This trainer could be trying to stop Necrozma if it's causing chaos, or perhaps they're trying to help it. They would have to navigate the mysteries of the Hall of Origin, which could be filled with ancient Pokemon knowledge and power sources. There could be other Pokemon guardians in the Hall of Origin that the trainer has to face or befriend while dealing with Necrozma.
There might be some legendary Pokemon that are guardians of the Hall of Origin. For instance, Arceus could be a key character, as it has a strong connection to the Hall of Origin. And then there could be other trainers who are either rivals or allies of the main trainer in the story. They could all have different motives regarding Necrozma and the Hall of Origin. Some might want to capture Necrozma for its power, while others want to protect it or the Hall of Origin from harm.
It could be an experienced Pokemon trainer who has already conquered many gyms and is now seeking a greater challenge in the Hall of Origin.
It could be about Melmetal's origin story within the Hall of Origin. Maybe it discovers a hidden power there or meets other rare Pokemon.
It can fit by exploring Flannery's backstory more deeply. Maybe her 'futa' nature is related to some ancient Pokemon power or prophecy. For instance, she could be the key to unlocking a hidden aspect of the Pokemon world that has been forgotten over time.
A young and curious trainer who has always been interested in the legends of Pokemon could be the main character. They might have heard rumors about the Hall of Origin and set out to find it.