One common trope is the 'opposites attract' within the harem. For example, a shy main character surrounded by more outgoing love interests. Another is the 'jealousy' trope, where the members of the harem get jealous of each other's interactions with the main character.
There's often the 'mysterious newcomer' trope in harem fanfiction. A new character enters the harem, shaking up the existing dynamics. This could be a new love interest or someone who has a connection to one of the existing members. It adds an element of surprise and new storylines to the fanfiction.
A 'found family' trope is quite common. Characters who were alone or estranged in the show form a close - knit family in fanfiction. For instance, Henry might bring together different characters who then become his new family. Also, the 'hurt/comfort' trope, where a character is hurt (physically or emotionally) and another character comforts them, is frequently used in Once upon a Time fanfics.
In Once Upon a Time fanfiction, a common trope for female OCs is the 'love interest' trope. She could be the love interest of a main character, like Hook or Charming. Then there's the 'mysterious stranger' trope. She arrives in Storybrooke or the Enchanted Forest with a mysterious past and unknown powers. Also, the 'redeemed villain' trope can be applied. Maybe she was once on the wrong side but then sees the error of her ways and joins the good guys in the fight against evil in the Once Upon a Time universe.
One popular trope is the 'forbidden love' trope. In 'Once upon a time', there are so many different storybook characters from different worlds and with different statuses, so a relationship between two characters who are not supposed to be together, like a princess and a villain, is quite common in the fanfiction. Another is the 'reunion' trope. Since many characters have lost their memories or been separated in the show, fanfiction often features them reuniting and falling in love all over again.
One popular trope is the 'destined to be together' for the main characters. It often involves a complex relationship between hunters and supernatural beings. Another is the 'found family' trope, where a group of misfits come together to fight against evil. And the 'hidden power' trope, where a character discovers they have a special ability they didn't know about before.
A common trope is the time - travel trope. Maybe a character from Once Upon a Time travels to the Descendants' world in the future or vice versa. Family reunions are also popular, especially when characters from the two shows discover long - lost family connections. And of course, the 'hidden power' trope, where a character discovers a new and unexpected power when they enter the other show's world, which they then have to learn to control.
One popular trope is the 'Enemies to Lovers'. For example, a hero and a villain character from 'Once Upon a Time' might start off hating each other but gradually fall in love. Another is the 'Soulmate' trope, where two characters are fated to be together, which adds a touch of magic and destiny to the story. And of course, the 'Hidden Relationship' trope is also common, as characters may have to keep their relationship a secret due to the circumstances in the 'Once Upon a Time' universe.
One popular trope is the fish - out - of - water. When the Descendants characters are brought into the Once Upon a Time world, they're like fish out of water. For example, the VKs (Villain Kids) from Descendants are used to the Isle of the Lost, and in Storybrooke or other OUAT settings, they have to adapt to a whole new set of rules and cultures. Another is the family connection trope. Since both shows have characters based on fairy tales, finding out new family relations between the two universes is a common and interesting trope. For instance, if a Descendants character is related to an OUAT character in a way not explored in the original shows.
Another common trope is the 'Time Travel' trope. Given the magical nature of the 'Once Upon a Time' universe, it's not surprising that many fanfictions use time travel as a plot device. For example, Emma or Hook might travel back in time to change an event that affects their relationship, or they could be sent to the future and have to deal with the consequences of what they see there in relation to their love for each other.
One common theme is love and relationships. Since the show has a lot of complex character relationships, fanfiction often explores different aspects of love between characters like true love's kiss and forbidden love. Another theme is redemption. Many characters in the show have a dark past, and fanfiction might focus on their journey towards redemption.
Family relationships are a big theme. Some fanfics focus on the family dynamics between characters like the Charmings and their descendants. There are also fanfictions that revolve around the theme of identity. Characters in 'Once upon a time' often have complex identities, and fan writers like to dig deeper into how these identities are formed and how they can change over time.