Often, original characters that are strong warriors are popular. For example, a human ranger who has extraordinary archery skills and a deep knowledge of the Northern lands. He could be a lone wolf type character who joins the fight against the dark forces.
One interesting storyline could be about a group of overlooked heroes in the North who form an alliance to fight off a new and unexpected threat that the main characters in the original LotR didn't face directly. They might discover ancient magic or weapons that change the course of the war in the North.
Gandalf is surely a popular character in such fanfiction. His wisdom and powerful magic would make him a central figure. He could be the one guiding the characters from both universes on how to deal with the new threats that emerge from the crossover. Hermione is also popular. Her intelligence and knowledge of magic could be very useful in this new combined world, and fans love to see her solve problems.
Some of the most popular original characters are those who are related to the main characters in unexpected ways. For instance, a long - lost relative of Harry Potter who shows up during the war with some crucial information or powers. There are also original characters who are healers with extraordinary skills, as they play an important role in treating the wounded during the war, and fans love to read about them.
There's often a character who is a powerful mage. This mage might be an orphan who discovers their powers and uses them to influence the war. Another popular type is the heroic commoner. This character rises from being a nobody to a key figure in the war, like a peasant who becomes a great warrior.
One popular original character might be a young, prodigious shinobi from a small village. They have unique abilities that set them apart, like the power to control the elements in a completely new way. Another could be a mysterious figure who shows up during the war and has a connection to an ancient shinobi clan that was thought to be extinct. And then there's the brave female shinobi who defies gender norms and becomes a respected leader on the battlefield.
Mrs. Thornton can be an interesting character to include. She has her own opinions and a strong presence in the original story. In a sequel, she could either be an obstacle or a support to the main characters, especially as the social and family dynamics shift and change.
Jon Snow is often very popular in these fanfictions as he was the King in the North in the show. His complex backstory and moral compass make him a great central character. Arya Stark is also quite popular as she could play different roles, from being an assassin protecting the King of the North to being his advisor in matters of politics. Another popular character is Tormund Giantsbane. His friendship with Jon and his wildling ways can add a lot of humor and action to the fanfiction stories.
Celestia is often a popular character. She's a powerful and regal figure, and in fanfiction, her leadership during the war can be a central focus. Another popular one is Twilight Sparkle. Her intelligence and magical abilities make her a key player in many war - related scenarios. Rainbow Dash also tends to be popular as she's brave and her flying skills can be useful in battle.
Well, the original main character from the story often remains popular in fanfiction. People like to continue his story and see how he fares in new situations.
Del Walker is another character that shows up a lot in fanfiction. His loyalty to JD and his role as a support in the team can be explored in different ways. Maybe a fanfiction could focus on a mission where Del has to make a tough decision to save JD or the rest of the team, showing his true character.