One idea could be a crossover with a popular RPG. The protagonist of Cave Story could find themselves in the world of the RPG, facing new enemies and having to learn different magic or combat systems. For example, if it's a Final Fantasy - like world, they might have to deal with complex job classes.
A crossover with a sci - fi game. Maybe the Cave Story world gets invaded by aliens from a space - faring game. The main character then has to use his skills and the unique weapons from Cave Story, like the Polar Star, to fight off these extraterrestrial threats. He could also team up with some of the sci - fi game's characters, who might introduce him to advanced technology like energy shields or teleportation devices.
One idea could be exploring the backstory of the Mimiga. Maybe there was a time before the events of the game when they had a more complex society. Another is to focus on Quote's life after the main story, like how he adapts to a more peaceful world. Or you could create a fanfic where an unknown Mimiga has special powers and becomes a key figure in saving the island from a new threat.
One idea could be to cross it with another game like Grand Theft Auto. The protagonist from Mafia 3 could find himself in the chaotic world of GTA, dealing with different gangs and their unique styles. He might have to adapt to the new city layout and rules.
A Goten and Harry Potter crossover could also be cool. Goten could be sent to Hogwarts by some magical accident. He would be the odd one out, not understanding the wizarding world at first. But his Saiyan strength could come in handy during the Triwizard Tournament, for example, when facing the dangerous tasks. Maybe he'd have to use his ki blasts instead of magic spells to get through certain challenges.
A great plot could involve the transfer of Personas between the two groups. For example, a character from Persona 3 accidentally gets a Persona that belongs in the world of Persona 4 and vice versa. This causes chaos as they have to learn to control and understand these new entities while also dealing with the reactions of their respective teams.
Maybe a crossover with a group of time - traveling scientists. They accidentally bring the Indoraptor to their time and have to figure out how to send it back while avoiding being hunted. This could involve a lot of scientific jargon and problem - solving. For example, they might need to study the Indoraptor's DNA to find a way to open a time - travel portal that it can go through. And along the way, they could learn more about the Indoraptor's behavior and how it evolved in its own time period.
One idea could be a crossover with Minecraft. The Terraria characters could find themselves in a Minecraft world filled with blocky landscapes and different mobs. They would have to adapt to the new building mechanics and find ways to combine their own abilities with the resources available in Minecraft.
One idea could be a crossover with the Mario universe. The Pikmin could help Mario collect power - ups in a new, Pikmin - themed world. For example, they could work together to defeat Bowser who has invaded the Pikmin planet.
One idea could be to cross it with a post - apocalyptic zombie world. The characters from Into the Badlands have to fight both zombies and their usual foes. They could use their martial arts skills in new ways, like using katana to slice through hordes of zombies.
A crossover with Spider - Man could be cool. Imagine Spider - Man trying to stop the Punisher from killing a bad guy. Spider - Man believes in rehabilitation while the Punisher thinks the only way to stop crime is to eliminate the criminals. This conflict could drive an interesting story.
One idea could be to cross the characters with another popular supernatural show's characters. For example, bringing in some characters from 'Buffy the Vampire Slayer' and seeing how they interact with the originals. Maybe Buffy could team up with Klaus to fight a new, powerful enemy that threatens both their worlds.