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 Michael could be a fascinating character for an Underworld Blood Wars fanfic. Even though his role in the movies has been somewhat limited, you could expand on his story. Maybe he has some latent powers that are yet to be fully explored. Also, the lycan characters like Raze can add a different perspective. Their loyalty to their pack and their fight against the vampires can be delved into more deeply.
Selene is a key character. She is a powerful vampire. Also, there are other important vampire lords who play significant roles in the power struggles.
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.
Anakin Skywalker is also an interesting character to focus on. His relationship with Obi - Wan is complex, and during the 'Deception' arc, his reaction to his master's supposed turn to the dark side can be explored in depth. How does he balance his loyalty to Obi - Wan with his duty as a Jedi? And how does it affect his own path towards the dark side?
One idea could be to explore the backstory of a minor character from the movie. For example, a young vampire who was turned during the wars but has unique powers. Another could be a love story between a vampire and a lycan that defies the age - old feud in the Underworld Blood Wars universe. Maybe they team up to stop a greater evil that neither side was aware of.
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.
The Sith Eternal cult members are really interesting to focus on in fanfiction. They have this mysterious and menacing presence in the movie. You could create a story about their origins, their goals, and how they managed to survive in hiding for so long. Also, General Pryde could be a character to explore. His loyalty to the First Order and his military strategies could be the basis for an exciting fanfiction story.
Wrecker is a character full of potential. His strength but also his more innocent side could be explored. Maybe he befriends a local on a planet they visit and gets into some comical yet heartwarming situations. Crosshair is also fascinating, especially if you write about his internal struggle, like his loyalty to the Empire and his past with the Bad Batch.
Kojou Akatsuki is definitely a popular choice. As the main character, he has a lot of potential for different storylines. His complex nature as a vampire yet still a high school student makes for interesting character development.