Maybe Godzilla, now human, discovers he has some remnants of his former powers. He could use these powers to protect a small town from other threats, all while trying to fit into human society. He would have to deal with the fear and suspicion of the townspeople at first.
One idea could be that Godzilla, as a human, has to hide his true identity while trying to understand human emotions. He might struggle with the concept of love and friendship, which are very different from his previous existence.
One idea could be a human scientist who discovers a way to communicate with Mothra and tries to convince her to team up with Godzilla against a new common enemy, like an alien invasion. The scientist could be in a race against time as the world is being quickly overrun.
Perhaps Godzilla in human form falls in love with a human. But he is constantly worried about endangering the person he loves because of his true nature. He has to battle his own kind, who see his relationship with a human as a betrayal, while also trying to keep his human love interest safe from the chaos of the kaiju world.
One idea could be that Casper has to learn about human emotions in a very short time. For example, he might experience love at first sight with a human girl and then struggle to understand jealousy when he sees her with another boy.
A plot could revolve around Damon trying to make amends for his past vampire deeds as a human. He could start by helping the victims of his previous vampire attacks or their families. Also, his love life would change. If he was in a relationship with a vampire or a human before, now as a human, new dynamics would emerge. He might worry about being too weak to protect his loved ones.
Perhaps there is a scientific experiment gone wrong that turns Sans into a human. He then has to hide his identity from the other monsters while he tries to find a way to reverse the process. In the meantime, he experiences human sensations like hunger and fatigue for the first time, which makes for some comical and also poignant moments in the fanfiction.
One idea could be that the Godzilla sees the human child in a dangerous situation, like a collapsing building during a battle, and decides to protect the child. It could then form an unlikely bond with the child, who is not afraid but rather curious about the giant creature.
Another plot could be that Damon becomes human again to be with a human he loves. But once he is human, he realizes that the world he thought he knew as a vampire is very different. He has to start from scratch in building relationships, getting a job, and learning how to live without his vampire powers. He also has to face the jealousy and hatred from some vampires who see his transformation as a betrayal.
A fanfiction could explore the idea of Shin Godzilla having a more sentient side. Maybe it tries to communicate with humans through some form of telepathy. The story could then revolve around the humans trying to understand what Godzilla is trying to say and if they can co - exist. For example, Godzilla might be warning about an upcoming natural disaster that only it can sense due to its connection with the earth.
A cool concept for 'human becomes twilight fanfiction' is a human who gets transported into the Twilight universe. This person could try to change the story's events. Maybe they befriend Bella and try to prevent some of the dangerous situations she gets into. Or they could end up falling in love with a minor character and create a whole new sub - plot. It could also be that they are turned into a hybrid of a vampire and a werewolf, which is a unique situation not really explored in the original series.
One idea could be a story where Godzilla and King Ghidorah have to team up against an even greater threat from outer space. This would force their natural enmity aside as they realize the common danger.