There could be some humorous interactions too. For example, Rocket Raccoon might make fun of the size of the Jaegers, comparing them to clunky machines. But then he'd be impressed when he sees them in action. Dr. Strange could use his magic to enhance the Jaegers' defenses or help in locating Kaiju. And the Pacific Rim characters would introduce the Avengers to their unique culture, like the importance of the drift and how it bonds the pilots together.
The Jaeger pilots would likely be in awe of the Transformers at first. They're used to big machines, but the Transformers are something else entirely. The Autobots would probably be friendly and helpful, trying to work with the humans. Optimus Prime could act as a sort of mentor to the pilots, teaching them about honor and sacrifice. The Decepticons, on the other hand, would see the Jaegers as a threat or something to be used for their own gain.
The RWBY characters could offer their knowledge of Aura and Semblances to the Pacific Rim pilots. In return, the pilots can teach them about the complex technology of the Jaegers. This exchange of knowledge can lead to new strategies in combat. Also, there could be some friendly rivalries between different teams, both from RWBY and Pacific Rim, which adds an interesting dynamic to their interactions.
One idea could be that the Avengers are called upon to help in the Pacific Rim universe when a Kaiju threat becomes too great for the Jaeger pilots alone. They could use their unique powers to assist in battles. For example, Thor could use his lightning to power up a Jaeger's weapons system. Another idea is that some of the Avengers become Jaeger pilots themselves. Iron Man could use his tech knowledge to upgrade the Jaeger technology. And Hulk could be a great asset in the physical combat against the Kaiju.
Godzilla himself is of course a key character. Focusing on his perspective could be really unique. Maybe we see his thoughts as he encounters the Jaegers and these strange new kaiju. We could explore his instincts and how he decides whether to fight or cooperate with the humans and their giant machines.
Tony Stark's cocky and brilliant personality would likely clash with Dwight Schrute's uptight and rule - following nature. Stark would probably make fun of Dwight's beet farming and Dwight would be constantly trying to assert his authority over Tony.
Naruto himself would be a great choice. His ability to quickly adapt and his large chakra reserves could be useful in powering and controlling a Jaeger. Also, his experience in combat situations would give him an edge.
The interactions would be complex. The Guardians are more of a ragtag group compared to the Avengers and Justice League. Star - Lord might try to charm Lois Lane, which could annoy Superman. Thor could have a friendly competition with Shazam over who has the better lightning - based powers. And Hawkeye could be in awe of Green Arrow's archery skills and ask for some tips.
First, you need to study both universes thoroughly. Know the characteristics of the Jaegers, their pilots, and the kaiju in Pacific Rim. For Godzilla, understand his powers and his different forms. Then, come up with an interesting concept like a crossover battle or a team - up. Next, create engaging characters, whether they are new or existing ones from the two universes.
New original characters created specifically for the fanfiction can also become popular. For instance, a character who is a Kaiju whisperer, someone who can understand and communicate with Kaiju. This character can bring a whole new dynamic to the relationship between humans and Kaiju in the fanfiction world.
One common theme is self - discovery. The oc characters often start out not fully understanding their place in the Pacific Rim world and then through their experiences, they figure out who they are. Another theme is friendship and teamwork, as they have to work with the existing characters in the Pacific Rim universe to fight the kaiju.