Palpatine could still be the main antagonist. Even if Mace Windu travels through time, Palpatine's long - term plans for galactic domination would still be a major threat. Windu would have to find new ways to outwit him in different time periods.
Darth Maul could also be a great antagonist. If Mace Windu travels back in time to when Maul was at his peak, Maul's thirst for revenge against the Jedi could make him a formidable foe. He might target Windu specifically as a high - ranking Jedi.
The dark side itself could be the antagonist. As Mace Windu travels through time, the ever - present allure of the dark side could be what constantly tries to corrupt him or thwart his attempts to change the past. It's an omnipresent force that can manipulate events and people to maintain the status quo of the Sith's power.
Another possibility is rogue Jedi who have turned to the dark side. They could be former comrades of Mace Windu, and he has to face them in combat. This would add an emotional layer to the story as he has to fight people he once considered friends and allies.
Voldemort would still be a likely main antagonist. After all, he is the big baddie in the Harry Potter series. Even with Mace Windu's training, Harry would still have to face Voldemort. However, there could also be new threats emerging from the combination of the Star Wars and Harry Potter universes. Maybe some Sith - like characters could be drawn to the power that Harry is developing under Mace Windu's tutelage.
Maybe Mace Windu travels to a time long before the Sith rule and learns ancient Jedi techniques that he then brings back to his own time. This could change the power dynamic between the Jedi and Sith in his era. He might learn how to better resist the dark side temptations that the Sith use against the Jedi.
Well, it could focus on Mace Windu's early days in the Jedi Order. Maybe show how he was trained, his first encounters with the dark side threats, and his rise within the Order.
In such fanfiction, it could lead to a completely different power dynamic in the galaxy. Maybe the Jedi would regain their prominence and prevent the rise of the Empire.
Mace Windu was a powerful Jedi Master. He was known for his purple lightsaber and his skills in combat. He was a member of the Jedi Council. He played a significant role in the Clone Wars, leading the Jedi in many battles against the Separatist forces. He was also involved in the discovery of the Sith Lord Palpatine's true identity, which ultimately led to his confrontation with Palpatine in the Chancellor's office.
Maybe Darth Vader. If the story is set at a time when Vader is at the height of his power, he could be the main obstacle in Mace Windu's path of revenge. Vader's loyalty to the Emperor and his own dark side powers would make him a formidable foe for Windu.
He could be a Force - ghost. His spectral form could guide Kanan or Ezra at crucial moments. For example, when they face a difficult decision regarding the Dark Side, Mace Windu's Force - ghost could appear and offer his wisdom based on his own experiences with the Sith.
Perhaps Mace Windu survived and was captured by the Empire. He manages to escape during the events of Star Wars Rebels. He then teams up with the Ghost crew. His presence would be a huge boost to their morale, as he is a powerful and well - respected Jedi. He could also help in the fight against the Inquisitors, using his skills to outwit them.
A new, unknown dark side entity could be the main antagonist. Maybe Revan's time travel creates a rift in the Force that awakens a powerful being that feeds on chaos and destruction. This being would oppose Revan because his actions of trying to fix the timeline would go against the chaos that the entity thrives on.