It could be Salem's forces. Since Ruby is now a Maiden, Salem would likely want to either turn her to her side or eliminate her to prevent her from being a threat to her plans.
There might be other rogue Huntsmen or Huntresses who are power - hungry. They see Ruby as a target because of her Maiden powers and want to take those powers for themselves. They could be constantly plotting against her and trying to ambush her at every turn.
Another possibility is that there are some ancient and dark entities that are awakened by Ruby's new - found Maiden status. These entities have been slumbering for ages and now see Ruby as a key to achieving their own malevolent goals. They might be able to manipulate events and people around Ruby to try and get her in their grasp.
The White Fang could be antagonists, seeing Ruby as a symbol of human - made abominations (in their view) with her cyborg body. They might target her to make a statement against humans.
A new group of Grimm hybrids created by Salem could be the main antagonists. These hybrids could be specifically designed to hunt down the wolf pack. They would have the intelligence of humans (from Salem's influence) and the ferocity of Grimm. The wolf pack, led by Ruby, would have to figure out the weaknesses of these new enemies and fight to protect themselves and the people of Remnant.
Yes, in some RWBY fanfictions, Ruby can be depicted as the Summer Maiden. Fanfiction allows for creative interpretations and alternate storylines, so it's not uncommon to see such a concept explored.
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.
I'm not sure specifically as I haven't read this particular fanfiction. But generally, in RWBY, Ruby is a key character. A fanfiction with this title might focus on Ruby's journey towards becoming a maiden, exploring her growth, new powers she might gain, and the challenges she'd face along the way.
One idea could be Ruby discovers she is the Winter Maiden in a moment of great danger. As she tries to control her new powers, she has to face off against Salem's minions who are after the power of the Maiden. Ruby's friends, Weiss, Blake and Yang, are initially shocked but quickly support her in her new role.
One great RWBY Ruby Maiden fanfiction is 'Ruby's Ascent'. It beautifully delves into Ruby's journey as a Maiden, exploring her new powers and the challenges she faces. The author does an amazing job of keeping Ruby's character true to the original while also adding unique twists to her story as a Maiden.
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.
One possible main antagonist could be a powerful Grimm lord that has emerged. This Grimm lord could be far more intelligent and organized than regular Grimm, and it sees Ruby's growing army as a threat to its plans of total domination. It could send out waves of Grimm to attack Ruby's army during its formation, trying to disrupt and destroy it.
One possible fanfiction could be about Ruby's journey as the Spring Maiden. She might struggle to control her new powers at first. For example, she could accidentally cause plants to grow uncontrollably during a battle, which both helps and hinders her team. As she learns to master her maiden powers, she could discover new ways to use them in combination with her scythe skills, making her an even more formidable fighter.