Godzilla is an obvious choice. The dynamic between Mechagodzilla and Godzilla can create intense and exciting stories. Also, Mothra could be included. Her gentle nature could contrast with Mechagodzilla's more mechanical and often aggressive nature. Another character could be King Ghidorah. The three - headed monster could form an alliance or be in direct conflict with Mechagodzilla in a fanfic.
You can try looking on fanfiction websites like Archive of Our Own (AO3). There are often many kaiju - related fanfics there. Just use the search function and type in 'Anguirus x Mothra' and it should show relevant stories.
Maybe in a post - apocalyptic world where kaiju are the dominant species. Anguirus is a loner, and Mothra is seen as a symbol of hope. Anguirus initially scoffs at Mothra's 'holier - than - thou' attitude. But when Mothra saves Anguirus from a deadly trap, Anguirus has a change of heart and they form an alliance to restore order in their world. This could involve many action - packed scenes and emotional moments between them.
Another theme is the search for identity. The female Mechagodzilla may be different from the traditional Mechagodzilla, and the fanfiction explores how she figures out who she is in the context of the kaiju world. She could be a hybrid of different technologies or have unique abilities that make her question her origin and purpose.
A Mechagodzilla x Mothra fanfic might explore their different natures. Mechagodzilla, being a man - made weapon of destruction, and Mothra, a symbol of nature and rebirth. The story could be set in a post - apocalyptic world where they have to work together to restore balance. Or perhaps it's a story of how their conflict shakes the very foundation of their world. There could be elements of human interference, like scientists trying to control Mechagodzilla while Mothra tries to protect the natural order.
One idea could be a crossover with the Transformers. Picture Mechagodzilla teaming up with Optimus Prime against a common enemy like an alien invasion force. They could combine their powers, with Mechagodzilla's brute strength and Optimus' leadership and advanced weaponry.
One idea could be a story where Mechagodzilla is reprogrammed by a young genius to protect the city instead of causing destruction. Another could be a time - travel fanfic where Mechagodzilla is sent back in time and has to face off against its earlier versions. Or perhaps a fanfic where Mechagodzilla teams up with other kaiju to fight a common, even greater threat.
Often, a theme of coexistence or conflict is present. Since they are so different, they can either find a way to live side by side in the story world or be in constant conflict. Also, the idea of redemption might be there. If Mechagodzilla is initially a force of evil, it could seek redemption through its interactions with Mothra. And there's the theme of the balance of the world. Their actions can tip the balance towards destruction or restoration.
Begin by brainstorming the setting. If it's a crossover with a fantasy world like Middle - earth, figure out how Mechagodzilla fits in. Will it be a mechanical monstrosity that the hobbits and elves are terrified of at first? Outline your story structure. Start with an interesting hook, like Mechagodzilla crashing into Middle - earth. Then build the conflict and resolution.