Hermione would be a likely supporter. She is always loyal to Harry and if he presented her with evidence of Dumbledore's wrongdoings, she would probably side with him. She has the intelligence to help Harry strategize and find ways to expose Dumbledore. For example, she could use her research skills to dig deeper into Dumbledore's past.
Ron could also be on Harry's side. Ron has a strong sense of loyalty and friendship. If Harry felt that Dumbledore had wronged him, Ron would be angry on Harry's behalf. Ron might use his family's connections within the wizarding world to help Harry spread the word about Dumbledore's actions. Also, Ron's courage would be an asset when it comes to standing up to Dumbledore's supporters.
Some of the younger students who looked up to Harry, like Neville, could support him. Neville has grown to be a strong and brave character. He might see Harry's actions as a way to make the wizarding world a better place. Also, those students who have felt wronged by Dumbledore's decisions in the past, perhaps in terms of school policies or safety, could come forward to support Harry in the court case.
Hermione might support Harry if she also sees some flaws in Dumbledore's leadership. She is very logical and if Harry presents her with solid evidence, she could side with him. For instance, if Dumbledore's actions are putting students in danger in some way, Hermione would want to protect her fellow students.
Creatures that have been wronged by Dumbledore's decisions in the past could side with Rumple. For instance, if Dumbledore once restricted the rights of house - elves, the house - elves might assist Rumple in his revenge plan, believing that Dumbledore needs to be taught a lesson.
In a 'Harry Potter gets revenge on Dumbledore' fanfiction, Harry could find out that Dumbledore knew about certain threats to his life but didn't do enough to protect him. So, Harry decides to get revenge by undermining Dumbledore's influence at Hogwarts. He might start by convincing the other students that Dumbledore isn't as infallible as they thought. He could do this by revealing some of Dumbledore's past mistakes. Then, he could work on getting the school governors to question Dumbledore's leadership, gradually chipping away at his power and status within the wizarding community.
Ron might support Harry at first. Since he's Harry's best friend, he would be on Harry's side without really knowing the full story. He'd probably offer to help Harry with some of his revenge plans, like distracting Ginny while Harry sets up a magical prank.
His friends from Hogwarts like Ron and Hermione. They are always by his side and would probably be more than happy to help Harry get revenge on the Dursleys. They might use their magic skills together to create some really interesting revenge scenarios.
Dumbledore, even if not in person, could have left behind some clues or resources that help Harry. Also, the Order of the Phoenix members who are still around and believe in Harry's cause. They have their own skills and connections that could prove useful. For example, Moody could teach Harry some advanced defensive magic techniques to use against the Ministry's security measures.
His friends like Ron and Hermione could be his allies. They have always stood by Harry and if Harry believes that Dumbledore has wronged him, they might side with him. Ron, with his loyalty and Hermione with her intelligence could help Harry plan and execute his revenge. They could use their knowledge of magic and Hogwarts to find ways to undermine Dumbledore's influence.
It could be the Ministry of Magic. They have often been shown to be corrupt and self - serving in the original series. If they try to manipulate Harry again, he might see them as the main antagonists and target them in his revenge.
His friends from Hogwarts like Hermione and Ron would most likely support him. They have always been by his side through thick and thin in the original story, so in the fanfiction, they would continue to support Harry in his fight against the Ministry.