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.
Perhaps an OC who is a time - traveling inventor. They come to Wonderland by accident and start to fix all the broken things in Wonderland, like the Mad Hatter's broken clock. Along the way, they get involved in the political intrigue between the Red Queen and the White Queen, using their inventions to either help or hinder each side depending on their moral compass.
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.
A great idea for an Once Upon a Time fanfic is to give a happily - ever - after to a character who didn't get one in the show. Take Neal Cassidy, for instance. You could write a story where he somehow comes back to life and reunites with his son Henry in a more complete way. Or you could do a time - travel fanfic where a character goes back in time to change a crucial event. This could lead to a whole new chain of events and relationships in the Once Upon a Time universe.
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.
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.
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.
One idea could be exploring the backstories of the lost boys in more detail. Maybe one of them has a hidden connection to a character from the outside world. Another plot could involve a new villain coming to Neverland who has a different kind of magic that challenges Peter Pan. Also, a story about Wendy's granddaughter returning to Neverland to find a lost family heirloom could be interesting.
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.
One idea could be a new agent arriving at Warehouse 13 with a mysterious past that turns out to be linked to an artifact that was thought to be lost forever. This new agent might unknowingly hold the key to controlling the artifact's power, causing a stir among the existing agents as they try to figure out how to handle the situation.
A love - interest plot for Gideon would also be interesting. He could fall in love with a character from a different realm within the Once Upon a Time universe. Their relationship would be complicated by the different rules and cultures of their respective realms. They would have to overcome many obstacles, like disapproving family members or the threat of war between their two realms.