A possible interesting plot could be that an ancient evil has awakened in the Once Upon a Time world. This evil is so powerful that it starts to corrupt the very fabric of the fairy - tale world. Hellboy is summoned through a magical portal because his unique nature, being part - demon, gives him some immunity to the evil's corruption. He teams up with Snow White and Prince Charming, who are initially skeptical of him but soon realize his value. Together, they embark on a journey to find the source of the evil and destroy it before it consumes both their worlds.
Imagine that a powerful sorcerer in the Once Upon a Time universe tries to use Hellboy's power for his own gain. He kidnaps Hellboy and tries to control him. But Hellboy is not one to be easily controlled. Meanwhile, the heroes from Once Upon a Time, such as Regina and Robin Hood, set out to rescue Hellboy. As they work together, they learn about each other's worlds and find common ground. In the end, they defeat the sorcerer and Hellboy decides to stay in the Once Upon a Time world for a while to help keep the peace.
It could work if the plot focuses on a common enemy. For example, a powerful demon that exists in both the Hellboy lore and could potentially invade the Once Upon a Time realm. Hellboy, with his knowledge of the underworld, teams up with the heroes from Once Upon a Time like Emma Swan and her crew. They could combine their powers, with Hellboy's physical prowess and the magical abilities of the Once Upon a Time characters to defeat this shared foe.
A possible plot is a crossover with the story of Beauty and the Beast. The characters from Wonderland might enter the Beast's castle. Alice could befriend the Beast and try to break the curse using her Wonderland knowledge. Or, a crossover with the Snow Queen's realm. The cold and ice of that world would contrast with the colorful and chaotic Wonderland. The characters would have to find a way to survive in the frozen land while also dealing with the Snow Queen's power. Also, a crossover with the Little Mermaid's underwater world. The Wonderland characters would have to figure out how to breathe underwater and interact with mermaids and sea creatures.
Well, a plot might involve Hellboy arriving in the Marvel universe and being mistaken for a villain at first. But then he meets Thor. Thor realizes Hellboy is not an ordinary foe and they start to communicate. Together, they discover that Loki has a plan to use an ancient artifact from Hellboy's world to gain power over all of Asgard and Earth. So they join forces with other Marvel heroes like Iron Man and Captain America to stop Loki.
One interesting plot could be that a dark force in 'Once Upon a Time' decides to use the power of the vampires and werewolves in Twilight to take over both worlds. The characters from both universes would have to team up to stop it. The heroes like Emma from 'Once Upon a Time' and Edward from Twilight would lead the charge. They would have to learn about each other's powers and weaknesses quickly to be effective.
One idea could be that Elsa's powers somehow link to a magic in the Once Upon a Time world. Maybe her ice powers are the key to unfreezing a character who has been trapped by a dark curse. For example, she could help break the spell on Emma Swan's parents. Another plot could involve Anna getting lost in the Enchanted Forest and having to team up with characters from Once Upon a Time like Snow White and Prince Charming to find her way back to Arendelle. A third idea is that the Evil Queen from Once Upon a Time tries to steal Elsa's powers to use for her own evil deeds, creating a big conflict between the two universes.
One idea could be a crossover with the world of Narnia. Harry and his friends could accidentally stumble into Narnia through a magical portal. They would then have to adapt to the new world, deal with the White Witch, and maybe use their own magic to help the Narnians. Another plot could involve a crossover with the Lord of the Rings. The Hogwarts students could be transported to Middle - earth to help in the fight against Sauron. They could use their unique spells and magical abilities in battles against orcs and other dark forces. A third idea might be a crossover with Alice in Wonderland. Harry could follow a strange magical creature down a rabbit hole and end up in Wonderland, where he has to solve the crazy riddles and deal with the mad characters while also trying to find his way back to Hogwarts.
First, understand the core traits of the characters from Once Upon a Time in Wonderland. For example, Alice is curious and brave. When crossing over, keep these traits but also show how they interact with the new world. If it's a crossover with a world full of strict rules, Alice's curiosity might get her into trouble more often. Second, give the new characters from the crossover world distinct personalities too. If it's a crossover with a steampunk world, the steampunk characters could be very logical and mechanical - minded. Third, create relationships between the Wonderland characters and the crossover characters. Maybe a friendship forms between Alice and a steampunk inventor because they both have a thirst for knowledge.
The Asset from Shape of Water could be transported to Hellboy's world. There, it would be seen as a strange and new kind of creature. Hellboy would have to decide whether to help it find its way back or keep it safe in his world. This could lead to a series of adventures as they face different threats together.
Some common themes in Once upon a time Hellboy fanfiction include the idea of family. Hellboy's adoptive family and his relationship with them are often explored. Another is the concept of destiny. Is Hellboy fated to be a hero or is there a different path for him? And then there's the theme of the supernatural world. Fanfictions might expand on the different creatures and beings in Hellboy's world, how they interact, and how Hellboy fits into this complex supernatural ecosystem.
A possible plot could be Belle's pregnancy being a target for the villains in the Once Upon a Time world. They might see the unborn child as a powerful asset or a threat. This could lead to Belle having to go into hiding and relying on unexpected allies for protection. It would also show her strength and determination to protect her baby at all costs. Additionally, the fanfic could explore how the different realms in Once Upon a Time react to the news of Belle's pregnancy. Some might see it as a sign of hope, while others could be envious or fearful.