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.
One idea could be a crossover where the characters from 'Harry Potter' visit the world of 'Legends of Tomorrow'. They could team up with the Legends to fight a common magical - time - related enemy. For example, Voldemort could be trying to manipulate time to his advantage and the combined forces have to stop him.
One great fanfic could be where Lemon has some sort of time - traveling adventure with the Legends of Tomorrow crew and meets Vixen. They could team up to fight a common enemy that threatens both their worlds.
Ray Palmer is also interesting. His scientific knowledge from Arrow combined with his experiences in Legends of Tomorrow make him a unique character. You could write about how he uses his tech - savvy skills to help the combined team in a crossover. He could be the one to come up with a plan to defeat a powerful enemy using his shrinking technology or other inventions.
Kira from The Dark Crystal could also be an interesting focus. If she were in the Harry Potter universe, her natural connection to nature and her strong will could make her a formidable ally. She could teach the Hogwarts students about the importance of nature in magic, which is something that is often overlooked in the wizarding world. Her skills with animals could also come in handy, like when dealing with magical creatures at Hogwarts.
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.
Neville Longbottom could be really interesting. He's usually so shy and awkward, but at the Yule Ball, he could have a moment of confidence. Maybe he asks a girl who no one expected him to ask, or he could show off some hidden dancing skills. Another option is Luna Lovegood. She might wear a really unique and eccentric dress to the ball, and her strange but endearing personality could make for some great interactions during the event.
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 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.
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.
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.