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 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.
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.
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.
Begin by choosing a concept that would make an interesting crossover. Let's say it's a 'Teenage Mutant Ninja Turtles' crossover. Consider how Hellboy would interact with the turtles in New York City. Maybe they have to fight a demon that's causing trouble in the sewers. Outline your story, including the characters' goals, the challenges they face, and how they will overcome them. You can also add some unique twists, like a special weapon that combines Hellboy's and the turtles' technology or magic.
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.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of Hellboy fanfictions. Just use the search function and type in 'Hellboy'. Another option is FanFiction.net, which also has a good number of stories. Additionally, some Hellboy fan communities on social media platforms might recommend or even share their own fanfictions.
Another concept is that a common enemy forces Naruto and Hellboy to team up. This enemy could be a powerful being that has the ability to disrupt both the ninja world and Hellboy's world. Naruto's jutsu and Hellboy's brute strength and Right Hand of Doom could be combined in creative ways to fight this foe.
One great example could be a crossover between 'Once Upon a Time' and 'Harry Potter'. Imagine the fairy - tale characters from 'Once Upon a Time' finding their way into the magical world of Hogwarts. Snow White could be a student in the Defense Against the Dark Arts class, learning spells alongside Harry and his friends. Another example is a crossover with 'The Lord of the Rings'. Characters like Regina might be intrigued by the power of the One Ring and get involved in the epic quest in Middle - earth.
I found a really fun crossover fanfic where the villains from Descendants team up with some of the forgotten villains from Once Upon a Time. It's called 'Villainous Alliances'. It shows how they would interact and form plans to take over both their worlds. The character development in this fanfic is quite good as it gives more depth to the Descendants' villains who might seem one - dimensional at first.
One common theme is the clash of different mythologies. When Hellboy crosses over with other universes, like the Norse mythology - based Thor universe, you get a mix of different supernatural elements. Another theme is teamwork. For example, when Hellboy teams up with characters from other franchises, they have to learn to work together despite their differences.