There could be a group of rogue Huntsmen who believe that the current system of protecting the world is flawed. They form a radical organization and start causing chaos. Their leader, who is a very powerful and charismatic character, could be the main villain. They would have their own set of ideals and be willing to do whatever it takes to achieve their goals, even if it means going against the established order.
A powerful sorcerer could be the main villain. This sorcerer has the ability to control minds and is using it to turn some of the Huntsmen and Huntresses against their comrades. He aims to take over Remnant and rule with an iron fist.
It could be a new character, a baker who has been corrupted by the power of the cookie. He wants to use it to take over Vale and Ruby has to stop him.
Maybe a rogue scientist from the dead civilization who was experimenting with dangerous powers and was thought to be dead. But now he has returned with a vendetta against the living, seeing them as unworthy of the knowledge he believes is his.
The main villain could be an older and more powerful version of Vlad. Maybe he has finally perfected his cloning techniques and created an army of half - ghost clones. He could be more devious and have a grand plan to take over both the human and ghost worlds.
In RWBY fanfic, it could be a new character introduced specifically for this purpose. The author might create a character with a special ability or connection that leads them to the discovery of the Relic of the Future. This character could be an outcast or someone with a mysterious past. They might discover it while on a quest to find their place in the RWBY world or while trying to solve a personal mystery. Once they find it, they become a central figure in the story as everyone else tries to either get the relic from them or help them understand and use it properly.
In addition to RWBY, other important characters like Qrow or Ozpin could also be focused on. Qrow has a lot of history and his own future could be intertwined with RWBY's. Ozpin, with his vast knowledge and secrets, might also play a crucial role in how they view the future. His wisdom could be used to help them understand what they see, or he might also be a part of the future they witness in unexpected ways.
It could be a rogue time - traveler. This villain has the ability to manipulate time and space, creating chaos across different timelines. They are bent on destroying the future warriors and the Z - fighters to ensure their own version of the future comes to pass, which is a future where they rule over all.
It could be a new teacher at the school who has very strict and old - fashioned views. This teacher doesn't believe in the progressive teaching methods that Riley and Maya want to implement, and constantly tries to undermine them. For example, the teacher might try to get their classes cancelled or their teaching materials removed.
Ruby could be a main character. Her speed and scythe - wielding skills would be really interesting to see in the Titanfall universe. She could quickly adapt to piloting a Titan and use her scythe in new and creative ways during battles.
Maybe an ancient evil spirit that has been awakened due to some mistake. It targets Ageha because she has a pure and powerful chakra. Naruto, being the hero he is, tries to fight the spirit, but he quickly realizes that he needs Ageha's help as well as the help of all his friends to defeat this powerful foe.