Who could be the main antagonists in a fanfiction where Anduin goes back in time?

Perhaps Orochimaru, if he somehow discovers Ino's time - traveling. He has always been interested in power and forbidden jutsu. He might try to use Ino's ability for his own gain, and if she resists, he would become an antagonist. He could also try to stop her from changing things that he wants to keep as they are for his own evil purposes.

Team Rocket could still be antagonists. Even if Ash goes back in time, they might be up to their old tricks of trying to steal Pokémon. Only this time, Ash has the advantage of knowing their future plans and can thwart them more easily.

The main antagonists could be the original Naruto's friends and allies. Since this is an evil Naruto, people like Sasuke, Sakura, and Kakashi would likely oppose him and try to stop him from whatever evil plans he has in the past.

Some of the older generations in Derry who are influenced by Pennywise's power but don't realize it. They might unknowingly work against Bill's attempts to change things. For example, those who are part of the town's power structure and are more concerned with maintaining the status quo than dealing with the real evil lurking beneath.

One possible antagonist could be the time itself. Since Harry is messing with the natural flow of time, there could be some sort of time - based magic or entity that tries to stop him. It might be a time - keeper or some ancient magic that protects the integrity of the timeline.

In some cases, it could be other time - traveling vampires or supernatural beings who have their own agendas. They might not want Bella to interfere with the past as it could disrupt their own plans or power bases. These antagonists could be more shadowy figures, working behind the scenes to thwart Bella's efforts.

The main antagonists in a 'Percy back in time fanfic' could be the followers of the Titans. They were numerous and loyal to Kronos. They would view Percy as an enemy since he is from a time when the Olympians are in power. Additionally, if Percy travels to a time when a powerful and evil demigod once lived, that demigod could be an antagonist. For example, if he goes back to a time when a demigod was on a path of destruction before being stopped, Percy's arrival might disrupt the events that were supposed to stop that demigod, and so the demigod would turn against Percy. There's also the possibility of some ancient sea monsters that were more prevalent in the past and were under the control of the Titans or other evil forces.

A young and inexperienced member of the vanguard could also be a main character. Their naivete in dealing with the past could create interesting situations and conflicts. For example, they might accidentally change a small detail that has big consequences.

It could be a time - traveling version of Orochimaru. He might have his own plans for the past and Sasuke's interference could put them at risk. So, he tries to stop Sasuke from changing the past in a way that would undermine his future goals.

It could be Danzo. In the original story, Danzo was a major factor in the Uchiha massacre. If Itachi goes back in time, Danzo would likely still be plotting against the Uchiha. His thirst for power and his extreme methods would put him at odds with Itachi's new attempts to save the clan.