Well, you could write a fanfic where a new type of kaiju is introduced into Kaiju Paradise. This kaiju has unique abilities that no one has seen before. The main character is a scientist or a researcher in the park who tries to study this new kaiju. Along the way, they face challenges from the park's management who don't want the public to know about this new discovery, and also from the kaiju itself which is very unpredictable.
To create engaging characters for Kaiju Paradise fanfic, think about their motives. Are they in the park because they are thrill - seekers or because they were forced to be there? Also, consider their relationships. A character could have a strained relationship with another character who blames them for a kaiju - related incident. And don't forget about their appearance. A character with a unique look, like a scar from a kaiju encounter, can be very memorable.
The Kaiju No. 8 manga is mainly about a character's struggle and growth in a kaiju-infested world. It has elements of action, adventure, and mystery. The plot keeps you on the edge of your seat with unexpected twists and turns.