A new character, a powerful sorcerer from a long - forgotten clan who believes the Fall Maiden power belongs to his people. He could be extremely knowledgeable about the ancient ways of magic and use that knowledge to try and take the power from the current Fall Maiden. He would be very different from the usual Grimm - fighting antagonists in RWBY.
Salem could also be an antagonist in this context. She is always interested in any power that could tip the balance in her favor. She might send her minions or even come herself to either corrupt the Fall Maiden or take her power directly. Her overarching goal of world domination would make her a constant threat to the Fall Maiden and those protecting her.
Cinder Fall herself could be a great antagonist. Since she has a history with the Fall Maiden power, she might want to reclaim it or prevent someone else from fully harnessing it. Her manipulative and power - hungry nature would make her a formidable foe.
One possible fanfiction could be about Ruby's journey as the Fall Maiden. She might struggle with the new power and the responsibilities that come with it. For example, she could face challenges from those who want to steal the Maiden's power for their own evil purposes. Ruby would have to learn to control the power quickly while still being true to herself and her values.
A great plot could be that Ruby, as the Fall Maiden, discovers that the power she has is connected to an ancient prophecy. According to the prophecy, she is the only one who can stop an upcoming apocalypse. But to do so, she needs to find and unlock the true potential of the Fall Maiden's power. Along this journey, she teams up with some unexpected characters, learns about the history of the Maidens, and battles powerful foes who are trying to stop her from fulfilling the prophecy.
Well, one possibility is that Ruby has to face many challenges to become the Fall Maiden. Maybe she has to fight against other powerful foes who also want the power. She might struggle with the new responsibilities and powers that come with being the Fall Maiden.
Another option is a rival village that discovers the potential of the shrine maiden's powers. They send their best ninja to kidnap her and force her to use her powers against the Hidden Leaf. This antagonist would be more of a political and military threat, as they are using the shrine maiden as a tool to gain an advantage in the shinobi world.
A new character created specifically for the fanfiction could be the main antagonist. For example, a powerful being from another dimension who is attracted to the chaos in Remnant caused by the combination of RWBY and Starkiller's presence. This character could have abilities that are a threat to both Starkiller and the RWBY characters, forcing them to work together in ways they haven't before.
Roman Torchwick would be an obvious choice. He already has a criminal background in the RWBY series. He could return with a new and more devious plan, perhaps teaming up with other villains to cause chaos.
Salem herself could be a great antagonist. She might be after the Summer Maiden's powers to add to her own strength or use them to achieve some evil plan. Another option could be a new character created specifically for the fanfiction, like a rogue huntsman who has turned to the dark side and wants the Maiden's power for his own gain.
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.
There could be an ancient evil entity that was sealed away long ago in Vacuo. Due to some recent events, it starts to break free. It corrupts the minds of some of the King's men and tries to use them to get rid of the King and RWBY, who are the only ones who can stop it from fully emerging and causing destruction.
It could be a power - hungry general from one of the warring factions. Someone who is willing to sacrifice anything and anyone to achieve victory.