One possible plotline could be that Shimo has some sort of special power that can either help or hinder Godzilla. For example, Shimo might be able to communicate with Godzilla in a way no one else can.
A plotline could involve Shimo being from a different dimension where they have a unique connection to Godzilla. When Shimo ends up in Godzilla's world, they have to figure out how to survive while also dealing with their strange connection. This could involve teaming up with Godzilla to fight off common enemies or trying to find a way back to their own dimension while Godzilla becomes an important part of that journey.
I'm not sure of specific 'shimo x godzilla fanfiction' works. But you can start by looking on popular fanfiction websites like Archive of Our Own. There might be some creative takes on the relationship between Shimo and Godzilla there.
A possible plot could be that Indominus Rex somehow gets teleported into Godzilla's world. Confused and angry, it starts attacking everything in sight. Godzilla, seeing this new threat, decides to take it on. Their battle could cause massive destruction, but in the end, they might reach a truce when they both realize there are bigger threats out there.
One possible plotline could be Miki Saegusa trying to tame Godzilla. She uses her unique abilities to form a bond with him, like a trainer with a wild animal. This could involve her spending time near Godzilla, getting him used to her presence, and gradually influencing his actions to be less destructive.
One possible theme could be survival. Given their nature as giant monsters, they might be fighting for survival in a world that is constantly changing. Another could be power dynamics, as Godzilla and the Female Muto have different levels of strength and abilities.
Power struggle is a likely theme. Godzilla and Destoroyah are both extremely powerful, and a fanfic could explore who is truly the stronger one.
A plotline might involve Stitch trying to reform Leroy. Since Stitch has his own journey of becoming more good - natured, he could try to do the same for Leroy. They could face challenges together and through those experiences, their relationship deepens. This could involve them facing other villains or threats in their world and relying on each other's unique abilities. It could also be a story about Leroy's redemption arc with Stitch as his guide, showing how their relationship evolves from animosity to a strong bond over time.
One possible plotline could be a romantic adventure where Will and Cornelia go on a quest together. They might face various challenges that test their relationship and their individual strengths.
One possible plotline could be that Henrietta, with her own set of skills and experiences, somehow gets transported to the Naruto universe. She might have to adapt to the ninja world, and Naruto could help her in this process while also learning from her in return.
In some 'batman x kagome fanfiction' plotlines, Kagome might be mistaken for a meta - human in Gotham. Batman starts to investigate her, and as he does, he discovers the truth about her world. This could lead to a series of adventures as they both learn from each other. Another idea is that a villain from one of their worlds somehow manages to cross over to the other. They then have to combine their skills and knowledge to defeat the villain. There could also be a plot where Kagome uses her knowledge of ancient magic to help Batman upgrade his technology in a unique way.
A possible plotline could be a time - travel scenario where Kaiba somehow travels back in time and meets Atem before he was sealed away. This could lead to a series of events where Kaiba tries to change the course of history regarding Atem. Another plotline might be that they are both trapped in a mysterious dimension together and have to rely on each other to escape, which forces them to set aside their differences and work together.