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.
First, re - watch the movie to get a fresh feel for the characters, the setting, and the overall plot. Then, pick a starting point. It could be right after the movie ends, or you could go back to a key moment in the movie and rewrite it from a different perspective. For example, start from when they first see the alien ship.
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.
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 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.
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.
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.
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.
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.
For tsuperhero fan fiction, you might consider writing a time - travel story. Have a superhero go back in time to prevent a major disaster that changed the course of their world. This could lead to all kinds of interesting plot twists. Another option is to write a story where the superhero loses their powers and has to rely on their wits and the help of ordinary people to get them back. It would be a great way to show the hero's character development outside of just using their powers. And don't forget about writing a story from the villain's perspective, showing their motives and why they do what they do.