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.
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.
Perhaps a story where a baby Kaiju is accidentally transported to a different dimension. In this dimension, it's the only Kaiju, and it has to adapt to a completely new environment. Meanwhile, the humans in the Pacific Rim world are trying to find a way to bring it back, not knowing how it will have changed in this strange new place.
One idea could be that the Jaegers are equipped with Transformer technology. For example, their power cores could be replaced with energon, giving them new abilities. Another plot could involve the Transformers teaming up with the Jaeger pilots to fight a common enemy that threatens both Earth and Cybertron. Maybe a new breed of Kaiju that can also assimilate Transformer technology.
One idea could be that Godzilla and the Jaegers team up to fight a new, even more powerful kaiju that is immune to their individual attacks. They have to learn to combine their strengths, like Godzilla's atomic breath with the Jaegers' high - tech weaponry.
A crossover with the Godzilla universe. Maybe the Jaegers are called in to assist when Godzilla is facing an especially powerful and numerous Kaiju horde. The Jaegers' advanced weaponry and strategic capabilities could complement Godzilla's raw power in this scenario.
One idea could be that Godzilla emerges in the world of Pacific Rim during a Kaiju attack. The Jaegers initially see Godzilla as another threat but then realize he's actually fighting the same enemy. They could team up, with Godzilla's raw power and the Jaegers' advanced technology combining to defeat the Kaiju once and for all.
There could be a Pacific Rim OC fanfiction where the OC is a new pilot. This pilot has a special ability like being able to sense Kaiju movements from a greater distance than normal. The story could revolve around their journey of proving themselves in the face of skepticism from other pilots and commanders in the Shatterdome.
One popular 'Pacific Rim fanfiction' story could be about the further adventures of the Jaeger pilots after the events of the movie. Maybe they encounter new and even more powerful Kaiju. Another could focus on the relationships between the different pilots, like a romantic relationship forming between two characters who previously only had a professional bond.
One interesting fanfiction could be about the backstories of the jaeger pilots. It might explore their lives before they became pilots, like their families and how they were selected. For example, a story could focus on a young pilot who had a difficult childhood but found purpose in the jaeger program.