Well, Hades and Emma Swan could be a popular pairing in fanfiction. Emma is the savior, and the idea of her being involved with Hades, the lord of the underworld, creates a great contrast. It's like the ultimate clash between light and dark. Also, Hades and Belle could be an interesting pairing. Belle is known for her love of books and knowledge, and her relationship with Hades could be about her trying to understand his world of the underworld, and him being intrigued by her different perspective.
One common theme is the struggle between good and evil. Hades represents evil, but in fanfiction, there could be a blurring of these lines. For example, he might do good deeds for self - serving reasons that end up having positive impacts. Another theme is family. His relationship with other characters who could be considered family in the Once Upon a Time world, like Zeus or other gods, can be explored. And then there's the theme of fate. Hades is often associated with death and the afterlife, so questions about whether characters can escape their fated deaths through his intervention can be a theme.
The most common pairing is probably Peter Pan and Wendy. In the original story, there's already a sort of connection between them, and in fanfiction, it's often explored in a romantic way. Another pairing could be Peter Pan and a mermaid. Since mermaids are part of Neverland, it makes for an interesting romantic scenario.
Hermione could be a popular character. In some fanfictions, she might accompany Harry to the Underworld and use her vast knowledge to help them solve problems. For example, she could be the one to decipher ancient Underworld texts that hold the key to their mission.
Percy Jackson is often a popular choice. His heroic nature and his relationship with Annabeth make him a great focal point. Fans love to explore his adventures further in the context of the House of Hades.
Snow White and Prince Charming are often popular. Their classic fairy - tale love story can be twisted and turned in interesting ways within the context of a cursed story. Emma Swan is also a favorite. Her strong - willed nature and her role as the savior in the original series make her a prime character for fanfiction writers to explore in a cursed scenario. Another popular character is Regina Mills. Her complex personality, with her history as the Evil Queen, offers a lot of room for creative exploration in a cursed 'Once Upon a Time' fanfic.
Ivy herself is often a very popular character. Since it's an ivy fanfiction, she's usually at the center of the story. Then there might be her love interests. If there's a romantic sub - plot, the male lead who has a relationship with Ivy would be popular too.
Percy and Annabeth are usually very popular in House of Hades fanfic. Their relationship is a big draw for fans, and many fanfics explore different aspects of it, like how they support each other in the dangerous underworld. Nico is also quite popular as his character has a lot of depth and mystery that fanfic authors like to dig into.
Sora and Henry. Henry is always looking for adventure and has a connection to the storybook world. Sora could show him more about different worlds and how to fight using the Keyblade. They could form a really cool mentor - mentee relationship.
Percy and Annabeth are often very popular. Their relationship is a central part of the series, and in fanfiction, people like to explore different aspects of it, especially during their trials in the House of Hades.
One of the popular vampire characters is probably Killian Jones (Captain Hook). In some fanfictions, he might be turned into a vampire or have some sort of vampire - related story arc. His complex character and his relationship with other characters in 'Once Upon a Time' make him a great candidate for vampire - themed stories.