One idea could be exploring the backstory of the Fall Maiden before she passed on the power. Maybe she had a secret love interest that was a threat to the order of Maidens. Another could be a story where the new Fall Maiden struggles to control her powers and has to learn from an unexpected mentor, like a character from the criminal underworld who has unique knowledge about the nature of magic.
The Fall Maiden could be a double agent, seemingly working for the good side but actually being manipulated by a powerful, unseen force. The story could focus on the discovery of this betrayal by her friends and how they try to save her from the clutches of the enemy. This would involve a lot of character development and exploration of the complex relationships within the RWBY universe.