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 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.
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 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 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.
Optimus Prime from Transformers would be an amazing addition. His leadership and combat skills would be a great asset. He could inspire the Jaeger pilots with his sense of honor and duty.
One possible plot could be that Naruto and his friends discover a way to enter the Pacific Rim universe. They could use their ninja skills to pilot the Jaegers in unique ways. For example, Naruto might use his shadow clone jutsu to operate different parts of the Jaeger simultaneously, giving it an edge in combat against the Kaiju.
In this crossover fanfiction, there could be a situation where a powerful Kaiju has some sort of ability that is affecting Godzilla's normal behavior. The Pacific Rim team has to figure out how to reverse this effect while also fighting off other Kaiju threats. They send in their most advanced Jaeger to try and communicate with Godzilla and form an alliance. Along the way, the pilots of the Jaeger learn about Godzilla's history and the ancient battles it has fought against similar threats.
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.
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 possible interesting plot could be that the Survey Corps discovers a titan that has some sort of connection to the Kaiju from Pacific Rim. Maybe the titan has a similar biology or origin. They could then team up with the Jaeger pilots to study it and figure out how to defeat both the titans and the Kaiju. The Jaeger pilots could teach the Survey Corps about their advanced technology, while the Survey Corps shares their knowledge of fighting in a more primitive, yet effective way against the huge and dangerous enemies.