One popular 'godzilla x ghidorah fanfic' is 'Clash of Titans: Godzilla and Ghidorah'. It has an interesting plot where they team up against a common enemy. The author does a great job of describing their interactions.
Another theme is the relationship between good and evil. Godzilla is often seen as a protector, while Ghidorah is more of a destroyer. Fanfics may play around with this perception, showing that there could be more to their natures. In some, they might even switch roles to some extent. This theme makes the stories more thought - provoking.
Godzilla Earth is not a fanfic. It has its own official production and creative process, distinct from fan-created works. It's a legitimate entry in the Godzilla canon.
I recommend Godzilla 2: King of Monsters to you. The plot is about aliens attacking and threatening Earth. At the same time, Godzilla fights with other giant monsters like Mosla, Ratton, and Kidora. In the end, Godzilla helps humans find little Godzilla and defeat the aliens with the other monsters. I hope you like this fairy's recommendation. Muah ~😗
For developing the plot, you could introduce a new, powerful enemy that neither Godzilla Earth nor the existing characters in the crossover world can defeat alone. So, they have to combine their strengths, like Godzilla Earth's raw power and the other characters' strategic thinking. As they fight together, they also face internal conflicts within their newfound alliance. Some may not trust Godzilla Earth completely, and this mistrust can create interesting sub - plots and character development. Eventually, through a series of events and battles, they overcome their differences and defeat the enemy. This could also lead to a changed world view for all the characters involved, and perhaps a new status quo for the fictional world.
One idea could be that Godzilla Earth teams up with a group of humans who have developed advanced technology to fight off an alien invasion. The humans initially fear Godzilla but soon realize they need his power.
You can try some popular fanfiction websites like FanFiction.net or Archive of Our Own. They usually have a wide range of fanfictions including those related to Godzilla Earth.
In a King Ghidorah x Mothra fanfiction, a possible plot could be that King Ghidorah is somehow injured and Mothra, being the more benevolent of the two, decides to heal him. As she heals him, she also tries to change his evil ways. This could be a story of redemption for King Ghidorah, and how Mothra's kindness can have a profound impact on even the most evil of creatures.
Well, a fanfiction could be set in an alternate universe where King Ghidorah and Mothra are the last two of their kind. They have to find a way to repopulate their species, but their natural instincts are so different. King Ghidorah is more about destruction and dominance, while Mothra is about life and harmony. This conflict could be the central theme of the story, exploring how they try to overcome their differences for the greater good of their species.