I think Deacon could be an interesting character for a 12 Monkeys fanfic. He has his own set of beliefs and goals that often clash with the main characters. A fanfic could explore his backstory and what led him to be the way he is. Also, the Pallid Man is quite intriguing. His appearance and his role in the events of the series give a lot of room for creativity in a fanfic. You could write about his relationship with the other villains or his own plans that we might not have fully understood in the original show.
Ruby is always a great choice. In the Mirrorverse, her innocence could be twisted in an interesting way, or she could be the one to bring light to a very dark Mirrorverse. Weiss is also interesting as her family's influence might be different, and she could have a completely different set of skills or abilities. Blake could have a unique story too, perhaps her faunus heritage is more pronounced or has a different meaning in the Mirrorverse.
Diana is definitely an interesting character to focus on. Her journey of self - discovery as a witch with powerful and unique abilities makes for great fanfic material. Another is Matthew. His complex vampire nature and his relationship with Diana are full of potential for further exploration. Also, Ysabeau is fascinating. Her long life and her role as Matthew's mother bring a lot of depth to the story.
I think Beetlejuice is the top choice. He has such a unique personality, full of mischief and a desire for chaos. But also, Delia, with her new - age beliefs and her somewhat out - of - touch nature, can be really fun to explore in fanfic. You can put her in all sorts of strange situations in the Beetlejuice world and see how she reacts. Additionally, the sandworm could be an interesting focus. It's this huge, menacing creature, but there could be more to it, like a hidden gentle side or a backstory about how it came to be in the underworld.
Nebula is an interesting choice as well. She has a complex relationship with Thanos and has been through a lot. In the aftermath, she could be a key figure in trying to find a way to undo what Thanos has done. Also, her unique skills and knowledge of different parts of the universe could play an important role in the fanfic.
Minato is always an interesting character. His intelligence and powerful jutsu make him stand out. In a fanfic about the parents, his interactions with Hiashi could be really engaging. Maybe they start off not getting along because of their different clans, but then they come to respect each other. Also, Kushina's feisty personality would be great to explore in relation to Hinata's more reserved mother. Their differences could create some really interesting scenes.
Vision is definitely an interesting character. His unique origin as a combination of JARVIS, the Mind Stone, and Tony Stark's creation makes for a lot of potential exploration. Wanda Maximoff is also great. Her powers are complex and her backstory of loss and revenge can be delved into deeper. And of course, Ultron himself. His view of humanity and his extreme measures to 'save' the world can be examined from different angles.
Obviously, Aslan and the White Witch themselves are the most interesting. Aslan represents all that is good and powerful, and exploring his thoughts and motives can be really fascinating. The White Witch, on the other hand, is the epitome of evil, and delving into her mind can be very engaging.
Selene is always an interesting character to focus on. Her strength and determination as a vampire make her a great protagonist. You can explore more of her past and her internal struggles during the blood wars. Another is David, with his complex relationship with Selene and his own ideas about the future of the vampire race.
I think Max and Chloe are the top choices. Max has so much to deal with post - game. Her powers, her decisions, and how they affected everyone around her. She's constantly evolving and trying to understand herself. Chloe, on the other hand, has been through so much trauma. Her relationship with Max is also a key factor. You can explore how they support each other or maybe face new conflicts. But also, Victoria could be an interesting character to focus on. She was a bit of a bully in the game, but in the aftermath, she could have a redemption arc or struggle with her own insecurities.
I think Chise and Elias are the most obvious but also the most interesting. Chise's journey from a broken girl to a powerful mage is fascinating. Elias, on the other hand, his struggle to fit into the human world and his love for Chise in his own unique way. But also, the side characters like Silky can be interesting. Her role as the housekeeper and her connection to Elias' past can be explored in a fanfic.