It could be the Death Eaters as a collective. They would see Harry as an even greater threat with his new skills learned from Mace Windu. They might band together more tightly and come up with new and devious plans to stop Harry. Also, there could be some sort of dark wizard or witch who is jealous of Harry's newfound abilities and decides to side with Voldemort just to bring Harry down.
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.
A 'Harry Potter trained by Mace Windu' fanfiction might involve a lot of cultural clashes at first. Harry is used to the wizarding world of Hogwarts with its own set of rules and traditions. Mace Windu comes from a completely different universe with the Jedi Code. However, as the training progresses, Harry could start to incorporate some Jedi wisdom into his way of dealing with the Dark Arts. He might learn to stay calm and centered in the face of danger, like the Jedi do, which could give him an edge against Voldemort and his Death Eaters.
The plot could also involve a power struggle of sorts. Anakin, being a powerful Force user, might initially resist Mace Windu's training methods. This could create tension between them as Mace tries to mold Anakin into a more Jedi - like figure. Meanwhile, other Jedi might be watching this relationship closely, either in support or in suspicion, which could add a layer of political intrigue to the fanfiction.
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.
Padmé Amidala could also be a part of it. Since she has a strong relationship with Anakin, she might notice the changes in him due to the training. She could either support or be worried about the new Anakin that is emerging under Mace Windu's tutelage. Additionally, some of the other Jedi Council members like Plo Koon or Ki - Adi - Mundi could be involved, perhaps observing Anakin's progress and having their own opinions on it.
It could be a deranged fan who has found a way to bring their dark fanfiction ideas to life. This person might be jealous of the original story and wants to create chaos in the wizarding world through their fictional concepts made real.
Perhaps some of the pure - blood wizard families who look down on house elves training a wizard like Harry could be antagonists. They might see it as a threat to the traditional wizarding hierarchy and try to stop Harry and the house elves, thinking that it's not proper for a wizard of Harry's status to be trained by 'mere' house elves.
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.
In a 'Harry Potter trained by Goku' fanfiction, the main antagonist could be an evil wizard who has allied himself with dark forces from other dimensions. This wizard could see Harry, with his new skills from Goku's training, as a major threat to his plans of world domination. He might have powers that are a mix of dark magic and some sort of energy manipulation similar to ki. Harry would then have to use his combined magic and Goku - trained skills to take on this powerful and devious enemy.
It could still be Voldemort. Even with Revan's training, Voldemort's power and his army of Death Eaters would pose a significant threat. Harry would have to use his newly acquired skills from Revan to finally defeat Voldemort once and for all.
It could still be Voldemort. Even though Harry is being trained by Ra's al Ghul, Voldemort remains the main threat in the wizarding world. His dark magic and his army of Death Eaters are still a major problem for Harry and his friends.