A great concept would be a fanfiction where Godzilla's transformation is linked to a human's emotions. For instance, when a particular human character experiences extreme love or rage, it triggers Godzilla's unexpected transformation. This could add a whole new layer of emotional depth to the story.
One idea could be Godzilla emerging in the world of RWBY and the Huntsmen and Huntresses having to figure out how to deal with it. Maybe they initially mistake it for a new type of Grimm.
One idea could be a story where Little Godzilla teams up with other baby kaiju to protect their island home from human pollution. They use their small but growing powers to clean up the mess and teach humans a lesson about respecting nature.
How about a Godzilla crossover with the Avengers? Maybe in a world where the Avengers exist alongside the Kaiju. Godzilla could be seen as both a threat and an ally. Iron Man could try to study Godzilla's power source while Thor might see him as a kind of primordial force. Captain America would be the one trying to keep everyone in line and figure out how to co - exist with this giant monster.
One idea could be a Godzilla crossover with King Kong. They could start off as enemies but then have to team up to fight a common, greater threat like an alien invasion. This would bring in the classic monster -vs- monster dynamic at first and then shift to an alliance against a new foe.
One idea could be to write a story from the perspective of a civilian caught in the middle of the Kaiju battles. Show their fear, their attempts to survive, and how they interact with the military trying to stop the Kaiju. Another could be a story about a Kaiju that has unique abilities not seen in the original lore, and how it changes the dynamic of the wars. Maybe it could be a Kaiju that can communicate with humans and tries to form an alliance against the more destructive Kaiju.
A Godzilla and Evangelion crossover would be cool. Godzilla could be the ultimate Third Impact scenario that the Evangelion units have to face. It would be a battle of titanic proportions with the mechs trying to take down the giant lizard using their unique weapons and Godzilla's raw power overwhelming them at times.
One idea could be that Godzilla somehow ends up in Equestria. The ponies are initially terrified but then realize Godzilla is more interested in protecting the land from some new dark force that has emerged there. Maybe Godzilla's size and power are a mystery to the ponies at first, and Twilight Sparkle tries to study him to figure out how he got there.
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.
One idea could be that Godzilla somehow ends up in the wizarding world. Maybe he's accidentally summoned by a young, over - eager wizard who was trying to do a powerful spell but misread it. The wizards then have to figure out how to deal with this huge, destructive creature. They might try to use magic to communicate with Godzilla and understand why he's there.
One idea could be that Godzilla arrives in the RWBY world due to a strange rift in dimensions. The Huntsmen and Huntresses initially see Godzilla as a huge threat and try to fight it. But then they discover that Godzilla is actually there to fight a common enemy, like a new type of Grimm that is immune to normal attacks but vulnerable to Godzilla's atomic breath.