Bellatrix Lestrange is also really interesting. Her extreme loyalty to Voldemort and her wild, unpredictable nature can add a lot of drama to a fanfiction. You could explore her relationship with her family, especially her sister Narcissa, and how that influenced her actions. Another interesting villain is Draco Malfoy. While he's not a full - blown villain in the end, his struggle between following his family's dark legacy and doing the right thing can be a great focus for a fanfiction.
I think Fenrir Greyback is quite an interesting villain for a fanfiction. He's a werewolf and his brutal nature and lack of remorse make him a terrifying character. You could write about his attacks on humans and wizards alike, and how the wizarding community tries to deal with him. Also, the idea of him having some sort of alliance with Voldemort and the Death Eaters, and what that means for the safety of the wizarding world, could be really exciting to explore.
Definitely Snape. His complex character with his love for Lily and his role as a double agent has so much potential for exploration through memories. Harry himself is also interesting as his memories can uncover more about his family and his own growth. And Dumbledore, his past is full of mystery and his memories could reveal many secrets about his actions and motives.
Leonidas from '300' would be a great character to focus on. Imagine his reaction to the magic of the Harry Potter world. He's a fearless leader, and seeing him interact with wizards and magical creatures would be fascinating. He might see the magic as a new weapon or a strange form of sorcery that he has to either embrace or fight against.
Luna Lovegood is an interesting choice. She has such a unique perspective on the world, and in the post - war era, she could be involved in exploring the more hidden and strange aspects of the wizarding world that were overlooked during the war. Maybe she discovers new magical creatures or lost spells.
Doctor Strange would be an interesting character to focus on in a Harry Potter MCU fanfiction. He already has knowledge of magic in the MCU, but it's a different kind of magic compared to that in Harry Potter. He could compare and contrast the two magics, and perhaps even learn new spells from the Harry Potter universe. His presence could also bridge the gap between the two universes in terms of magic.
Hermione Granger would also be an interesting character to focus on. She's extremely intelligent and curious. In a crossover, she would be the one trying to figure out how The Flash's powers work from a scientific and magical perspective. She might even try to find a way to combine magic and the Flash's speed for some greater good. Her inquisitive nature would make for some great interactions with The Flash and would add a lot of depth to the crossover story.
I think it would be really interesting to focus on Hermione. She's so intelligent and curious. In Middle - earth, she could study the different magic and lore there. She might be the one to figure out how to combine the magic of the two worlds. For example, she could try to find a way to use the magic of Middle - earth to defeat Voldemort or help the Fellowship in a different way.
Harry Potter himself could be a great focus. Imagine him being introduced to the Halliwell sisters' way of life. His sense of justice and determination would be put to the test in a different magical context, and his relationship with Piper could develop in interesting ways as they learn from each other's magical experiences.
Harry Potter himself would be really interesting. He's used to facing dark wizards and magical threats, but the survival horror aspects of Until Dawn would be a whole new challenge for him. His courage and determination would be put to the test in a different way, like having to make quick decisions in a life - or - death situation without relying solely on his magic.
I think Hermione would be really interesting. She's so intelligent and resourceful in the 'Harry Potter' world. In a fanfiction with 'Legends of Tomorrow', she could use her vast knowledge of magic to help the Legends understand and deal with magical threats they encounter during their time - traveling adventures. She might be able to figure out how to combine magic with their technology in ways no one else could.
Well, you could write a fanfiction where the villains from different eras of the Harry Potter world team up. For example, Grindelwald and Voldemort joining forces in an alternate timeline. It would be really interesting to see how their different styles of dark magic would combine and how the heroes would deal with it. You could also focus on a single minor villain like Pius Thicknesse and expand on his character, perhaps giving him a more complex motivation for joining the Death Eaters than what was shown in the books.