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.
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.
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.
Another interesting plot could be Loki falling in love with a character from Once Upon a Time. Maybe he has a relationship with a princess or a powerful sorceress, and their love story is filled with magic, drama, and challenges as they come from different worlds.
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.
One idea could be exploring Gothel's backstory before she became the villain we know. Maybe she had a tragic love in her youth that turned her bitter. Another plot could involve Gothel teaming up with an unexpected character from the Once Upon a Time universe, like a hero who thinks they can reform her. A third option is to create a story where Gothel has a daughter of her own and we see how she raises her in contrast to how she treated Rapunzel.
One idea could be to create a story where a new character, an aspiring actress, befriends Rick Dalton and Cliff Booth. She gets involved in their crazy Hollywood adventures and helps Rick revive his career in an unexpected way, like by introducing him to a new type of movie role that suits his acting style better.
One plot idea could be that the OC is a lost girl from another dimension. Peter Pan discovers her and brings her to Neverland. She has unique powers that both fascinate and worry the Lost Boys. Another idea is that the OC is a mermaid who can transform into a human on land. She meets Peter Pan on the shore of Neverland and they embark on adventures together, with her mermaid knowledge helping them in various situations. And a third idea could be that the OC is a fairy who has been banished from her own kind. Peter Pan takes her in and she uses her fairy magic to help him against Captain Hook.
A possible plot could be that Drizella is cursed, and Henry, being the son of Snow White and Prince Charming, has a unique connection to the magic that can break the curse. They embark on a quest to find the ingredients for a potion to break the curse. Along the way, they meet old characters from the Once Upon a Time universe who either help or hinder them. They also have to deal with their own personal issues. Drizella has to confront her past and her insecurities, while Henry has to prove himself as a hero without relying too much on his family's legacy.
A really interesting plot could be that an ancient curse has been placed on both Emma's Storybrooke and Elsa's Arendelle. The curse is slowly draining the magic from both places. Emma and Elsa team up, using their combined magic knowledge and their strong wills. They travel to different magical realms to find the source of the curse and break it. They encounter many magical creatures and face various challenges that test their friendship and their magic skills.