Well, for a Shin Godzilla crossover fanfic, it could cross over with the Godzilla: Planet of the Monsters universe. Maybe the events in that universe lead to the emergence of Shin Godzilla in a different way. Or it could cross with the Power Rangers universe. The Rangers could use their powers and zords to fight Shin Godzilla, which would be a really cool scenario with lots of action and drama.
One idea could be that Godzilla arrives in the RWBY world due to a strange dimensional rift. The Huntsmen and Huntresses initially see it 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.
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.
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.
Start by choosing a crossover concept that interests you, like a Shin Godzilla and Transformers crossover. Next, establish the rules of this new combined universe. Will Godzilla be able to damage the Transformers? How will the Autobots or Decepticons view Godzilla? Then, start writing the beginning of the story. You could start with a mysterious event that brings Godzilla into the Transformers' world, like a space - time rift. From there, develop the plot by showing how different characters respond to this new situation and how their goals might change.
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.
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 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.
One interesting concept could be a Godzilla crossover with other giant monster franchises like King Kong. It could lead to epic battles and new storylines where they have to team up or fight over territory. Another concept might be a crossover with a superhero universe. Imagine Godzilla facing off against the Avengers or Justice League, which would require different strategies from both sides.
Another concept is that Godzilla somehow ends up in the Rosario Vampire universe. This causes chaos as the local creatures and humans are not used to such a huge and powerful being. The main characters from Rosario Vampire then have to figure out a way to either send Godzilla back to his own world or find a way for him to co - exist in theirs.
One idea could be a Godzilla crossover with King Kong. They could have an epic battle and then team up to fight a common enemy like a giant alien monster. Another could be a crossover with the Power Rangers. Imagine Godzilla helping the Rangers take on a really powerful villain. Or a crossover with the Transformers, where Godzilla and the Autobots have to deal with a threat that endangers both their worlds.