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.
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.
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.
I think the Emperor could be an interesting character to focus on. In 'Return of the Jedi Redux' fanfiction, we could explore his backstory more. What made him turn to the dark side in the first place? And are there any weaknesses in his seemingly all - powerful grip on the Empire? Also, the Ewoks could be given more depth. Instead of just being the cute but fierce forest dwellers, they could have a more complex culture and history that plays into the story.
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.