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.
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.
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.
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.