I think Caradoc Dearborn is a very underrated character for a First Wizarding War fanfic. We don't know much about him, so you have a lot of creative freedom. You could make him a key figure in some secret Order of the Phoenix missions. Also, Benjy Fenwick could be a great focus. His death was brutal, but his life before that could be explored. Maybe he had some special magical abilities that were overlooked. And don't forget about Marlene McKinnon. She was a strong witch, and a fanfic could tell the story of her fight against the Death Eaters from her unique perspective.
Blister is an underrated character. He has a unique look and his skills as a duelist could be explored more in a fanfic. Maybe a story could be written about his past, how he got into dueling, and what his goals are outside of the main events in the show. He could be given more depth and a more complex backstory, perhaps he has a family that he is trying to support through his dueling career.
Ser Davos Seaworth is often overlooked in some aspects. He has a great backstory and a strong moral compass. A fanfic could explore his early days as a smuggler more in - depth, or what he would do if he had to make a choice between his loyalty to Stannis and a new opportunity that could benefit the common people more. And then there's Bronn. He's always been in the background for the most part. A fanfic could put him in a situation where he has to decide whether to stay a sellsword or take on a more noble role, perhaps by taking over a small holdfast and ruling it in his own rough - and - tumble way.
Gauche Adlai is another underrated character. His Mirror Magic is really interesting. A fanfic could center around him and his relationship with his sister. Maybe he goes on a journey to find a way to completely bring his sister back to life, and in the process, he discovers new aspects of his magic that he didn't know before. He could also face some moral dilemmas about using his magic in extreme ways for his sister.
The guards at the Manyuu clan's gates are often overlooked. They are in a unique position, being the first line of defense and having to deal with all kinds of people coming and going. A fanfic could focus on their daily lives, the challenges they face, and how they interact with the more important members of the clan. Also, there's that one quiet librarian in the clan's library. She has access to all the ancient scrolls and books. A story could be written about her discovery of a long - forgotten prophecy that could change the fate of the Manyuu clan and how she decides to act on it.
Another underrated character is Kenshin Himura from Rurouni Kenshin. His vow to not kill again while still being a powerful swordsman could create interesting moral dilemmas in a Jump Force fanfic. He could interact with other characters who have different views on combat and justice.
Mantis. She has the power to manipulate emotions, which is a very interesting ability. In fan fiction, she could use this power to manipulate Thanos in a different way than what we saw in the movie. Maybe she could make him feel extreme guilt about his actions, causing him to hesitate at a crucial moment.
The Caragors are often overlooked as just mounts in the game. But a fanfiction could be written from their perspective. Imagine a story about a Caragor that has a special bond with a ranger, and how they fight together against the forces of evil. Also, the Gravewalkers are underrated. They have this spooky and mysterious element. A fanfiction could delve into their origin, what they are really made of, and their role in the grand scheme of the Shadow of War universe. It could be really interesting to see how they interact with other characters and how they came to be in Middle - earth.
Hot Pie is a character that could be really interesting. He's just a simple baker's boy, but he has his own dreams and fears. You could write about his adventures after leaving Arya, how he tries to make a name for himself with his baking skills in different parts of Westeros. Another one is Podrick Payne. He has shown loyalty and courage, and a fanfic could explore his growth from a young squire to a more confident knight - like figure.
Hawkeye. He's often overlooked but could be really interesting in a fanfic. He could have a solo mission during the events of Infinity Wars where he discovers a secret about one of the Infinity Stones that no one else knows. Maybe he has to use his archery skills in a completely new and creative way to protect this knowledge from falling into the wrong hands.
The servants in the castle, like Lumière and Cogsworth, could be explored more deeply. Their backstories before the curse could be interesting. For example, what were their relationships like? And how did they come to work in the castle?