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.
Maybe the house elves train Harry in the art of stealth. Since they are so good at moving around unnoticed in the wizarding world. They could pass on those skills to Harry, which would help him greatly in his battles against Voldemort and his Death Eaters. Harry could then use this new - found stealth to gather intelligence or surprise his enemies.
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.
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.
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.
Dobby would likely be one of the important house elves. He has shown great courage and a willingness to go against the norm to help Harry in the original series. In this fanfiction, he could be the leader among the house elves in the rescue mission.
Dobby is often very popular in fanfiction. Since he was already a well - developed character in the original series, fanfiction writers like to expand on his story further. He might be shown having more adventures or getting into different kinds of trouble. Another popular one is Kreacher. Writers may explore his complex personality more deeply, perhaps showing a side of him that was not fully explored in the books, like his loyalty to the Black family in new and interesting ways.
It could be a dark wizard who wants to take over the toy house for its magical properties. Maybe he believes that there are powerful artifacts hidden within the toys that can make him invincible.
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.
Another possibility is a group of Hogwarts professors who are very conservative in their views. They might believe that house elves should remain in their subservient positions and would try to stop Harry from making any changes. They could use their influence within the school to make things difficult for Harry and her friends as they try to advocate for the rights of house elves.