There could be a group of fanatical cultists who believe that the Maiden powers should be used in a different way than the Summer Maiden intends. They might try to capture her and force her to use her powers for their own twisted beliefs. This group could be made up of humans who have been corrupted by their own greed for power and see the Maiden as a tool to achieve their goals.
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.
Cinder Fall could also be an antagonist. Since she is already involved with the Maiden powers in the RWBY series, she might see the Summer Maiden as a threat or as a source of more power. She could try to manipulate situations to either defeat the Summer Maiden or steal her powers. Additionally, the Grimm could be used as antagonists. A horde of powerful Grimm could be sent after the Summer Maiden, either by Salem or by some other force that wants to see her fall.
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.
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.
A rival law firm could be the antagonists. They might try to steal Joe's clients or undermine his cases in unethical ways, like spreading false rumors about his legal abilities.
A group of rogue ninjas who have made the island their base could be antagonists. They might see Naruto as a threat to their operations on the island.
A powerful wizard who wants to use the 'lady of darkness' powers for his own evil gains could be an antagonist. He might try to manipulate her into doing his bidding.
In RWBY fanfiction, if Ruby is the Summer Maiden, there are several implications. Firstly, from a power - standpoint, she would have access to a whole new set of abilities. This could range from elemental powers related to summer, like heat or growth manipulation, to enhanced speed or strength. In terms of the story's plot, it could lead to new conflicts. Enemies who are seeking the power of the Maiden would target Ruby more directly. It could also lead to internal conflicts within Ruby herself. She might question if she is worthy of this new power and how it will change her role within her team. Her relationships with her friends and family could also be affected. Her friends may feel a sense of pride in her new status, but also worry about the danger it brings. Her family members, especially those with ties to the Maidens, might have different reactions, such as trying to protect her or guide her in using her new powers.
The Death Eaters could be antagonists. Even though Regulus turned against them, they might not forgive his betrayal and try to harm him or the school where he teaches. They could send spies or launch attacks to get revenge.
The other Death Eaters could be antagonists. They might not trust Snape completely and try to undermine him. For example, Bellatrix Lestrange could suspect Snape's loyalty to Voldemort and try to expose him while he's alone, which would put him in a very dangerous situation.
Some of the remaining members of the Akatsuki who still hold grudges against Sasuke or see the baby as a threat to their plans. They might target the baby to get revenge on Sasuke or to use it as a bargaining chip. For example, if there are any Akatsuki members who were loyal to Itachi and don't like Sasuke's new life with Naruko, they could cause trouble for the family.
A group of traditionalists in Akeno's world could be antagonists. They might not approve of the idea of a harem or of an outsider like Naruto having so much influence. So they could try to break up the harem and drive Naruto out of their world by using political means, like spreading false rumors about Naruto and his friends.