One idea could be exploring the backstories of the characters more in - depth. For example, fleshing out how Alex became so good at gaming before the alien invasion. Another could be creating an alternate ending where the group doesn't succeed in saving the world right away, but has to embark on a second, more dangerous mission.
In a Rim of the World fan fiction, you might consider a time - travel aspect. Suppose one of the characters accidentally travels back in time to before the alien invasion and tries to warn everyone. This would create a whole new set of challenges and opportunities. You could also write about the characters having to deal with internal conflicts within their group, like jealousy or power struggles, which would add more depth to the story.
One idea could be to explore what would happen if April discovered a new hidden power within herself. Maybe she could use it to save her world from an even greater threat than the ones she already faced.
Another interesting angle is to focus on a character who has the ability to communicate with phantoms in a unique way, like through music or dreams. For example, a character could use their musical talent to soothe the more aggressive phantoms and form an alliance with them against a common enemy.
One interesting plot could be a group of civilians forming a resistance against an occupying force during World War 3. They might use guerilla tactics and their knowledge of the local terrain to fight back.
One idea could be that the RWBY characters are Jaeger pilots. Ruby could use her speed and Crescent Rose in the cockpit to fight Kaiju. Another is to have Grimm as a new type of threat in the Pacific Rim universe, and the teams have to adapt their fighting styles.
One idea could be a crossover with the Transformers universe. Picture Jaegers and Transformers teaming up to fight a common enemy. Another could be a crossover with the Godzilla franchise, where the Jaegers have to deal with Godzilla while also facing their usual Kaiju threats. A third idea might be a crossover with the Star Wars universe, perhaps with Jaegers having some sort of technology exchange with the droids or the Jedi using their powers in the context of the Pacific Rim battles.
Another back story might be that of a colonist who was part of a failed utopian experiment. Their colony was supposed to be a paradise, but due to mismanagement, internal conflicts, and external threats, it fell apart. Now, they are in the Rim World, trying to find a new place to call home and start anew.
Maybe a story about a young character trying to break into the fashion industry in Bay City. They face challenges like competition and family drama, all while being part of the Another World universe. This could involve interacting with established characters in new ways, like getting advice from a veteran businessperson in town.
One idea could be an OC who is a brilliant but unorthodox engineer. They come up with new Jaeger designs that are initially scoffed at but prove crucial in a battle. For example, a Jaeger with the ability to split into multiple smaller units for more flexible combat.
Well, you could write a fan fic where the relationships between the characters are reversed. So if in the original there are certain rivalries, in the fan fic they become allies and vice versa. Also, a story about a character who loses their power but discovers a new way to be strong within the 'unordinary' world would be interesting. Or how about a fan fic that takes place in a parallel dimension within the 'unordinary' universe where the rules of powers are slightly different?
Some interesting ideas for Riverdale fan fiction could be a crossover with another popular teen drama. Imagine the characters of Riverdale meeting the characters from 'Stranger Things'. It would be really cool to see how they interact. You could also write a fan fiction that focuses on the future of Riverdale. What will happen to the town and its residents ten years from now? Will Archie still be in town? How will Veronica's business empire develop?