I think Wylie could be one. He has so much potential but often gets overshadowed by the main characters. In fan fiction, his relationship with other characters, like his family ties, could be explored more, and he could be given his own adventures that showcase his unique skills and personality.
Tam is a character with a lot of mystery. He and his sister Linh are interesting, but Tam often gets overshadowed by Linh's more obvious power. Fanfiction could explore his inner thoughts and his past more. For example, a story about his time before he joined Sophie's group and what made him the way he is. And then there's Keefe's mom. She's only briefly mentioned, but she could be a really interesting character to focus on in fanfiction. Maybe she has her own secrets and plans that could change the whole dynamic of the Lost Cities if they were revealed.
One idea could be exploring the backstory of a minor character in more depth. For example, the character's family history and how it might be related to the overall mystery in the Keeper of the Lost Cities world.
The house staff could be underrated characters. They've been around the Hawthorne family for a long time and surely know a lot of secrets. Focusing on them could add a whole new layer of intrigue to the story.
One underrated character is Prentiss. He's often just in the background in the original, but in fanfiction, he could be the hero. Maybe he discovers a hidden power within himself that helps Peter and the gang. Another is Ted. He could be given more spotlight by having his own backstory explored, like how he ended up on the ship in the first place.
The Genovian citizens. In fanfiction, individual citizens could be highlighted. For instance, a young Genovian girl who looks up to Mia as a role model. We could follow her journey of trying to be like Mia, and through her eyes, we can see how Mia's actions as princess impact the people of Genovia on a more personal level.
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?
Donkey's dragon girlfriend could be more prominent. In fan fiction, she could have her own set of challenges that Shrek and Fiona help her with. For example, she might be having trouble fitting into the ogre - friendly world, and Shrek and Fiona could offer their support and advice.
Since it's an extra stories collection related to 'Keeper of the Lost Cities', it should feature characters like Sophie. She is a very important character in the main series. There might also be Fitz, Keefe, and other members of their group. These characters are usually involved in all kinds of adventures and mysteries in the world of the Lost Cities. Their different personalities and abilities play a big role in the stories, and I assume they will continue to be central in the extra stories as well.
Sophie Foster is one of the main characters. She's the girl with special abilities.
Antoine could be a great character to focus on more. In Sonic SatAM crossover fanfiction, he could have a character arc where he overcomes his cowardice in a new and unique way. Maybe in a crossover with the Archie Sonic comics universe, he could learn some new combat skills from the more battle - hardened characters there.