Sanya V. Litvyak is often a popular character. Her calm and collected nature, along with her powerful abilities, make her interesting to write about. Fans like to explore her backstory, especially her time in her homeland before coming to fight in the war.
One idea could be exploring the backstories of the individual witches more in - depth. For example, how they first discovered their powers and what their lives were like before joining the fight. Another could be a story about a special mission that the witches have to undertake behind enemy lines, filled with danger and unexpected alliances.
Hermione Granger could be one of the main witches. She's extremely intelligent and powerful, and in the fanfiction, she might be involved in some secret magical research or adventures.
Fiona Goode. She is so powerful and complex. Her actions, both good and bad, make her stand out. She has a strong personality and her struggle with her own powers and age is really interesting.