Sirius Black could also play a role. Even though he is dead in the original story, in a fanfic he could appear as a sort of spirit guide. He was always one to question authority and do things his own way. So he could encourage Harry to defy Dumbledore if he believes Dumbledore is wrong, based on his own experiences with the wizarding world's power structure.
Luna Lovegood could be a supporting character. She has an open - minded and unique perspective. She might support Harry in his defiance, understanding his reasons when others don't. Maybe she helps him see things from a different angle or provides some crucial information that validates his decision to defy Dumbledore.
One possible plot could be that Harry discovers some dark secret about Dumbledore's past actions and decides to oppose him. For example, if he finds out that Dumbledore had some sort of deal with the Dark Arts that he hid from everyone. This would make Harry feel betrayed and drive him to defy Dumbledore.
Hagrid could be a great supporting character. He could bring some interesting magical creatures for Dumbledore and Harry to interact with. For example, he might bring a baby dragon that Dumbledore could use to teach Harry about the care and danger of such powerful creatures.
The Weasley family as a whole might also support Harry. They have always treated Harry like one of their own. Arthur Weasley, being an open - minded wizard, might understand Harry's perspective. Ginny, who is very close to Harry, would surely be on his side as well. They would see Harry as more than just a student defying a headmaster, but as someone fighting for what he believes is right.
In such a fanfiction, one possibility could be that Harry Potter defies Dumbledore over the use of certain dark magic items. Maybe Harry believes that Dumbledore is being too cautious and that these items could be used for good in the fight against Voldemort. For example, he might think that the Elder Wand could be harnessed in a different way than Dumbledore anticipates.
Hermione could be a likely helper. She's smart and resourceful, and she would likely believe Harry if he told her his reasons for escaping Dumbledore. She could use her vast knowledge of magic and spells to create distractions or to find ways to bypass Dumbledore's security measures.
Hermione would be a great supporting character. She could be the one who first notices Harry's interest in becoming a girl and offers her support. She might share her knowledge of magic that could help Harry in his transformation, like spells for changing appearance or potions that can affect one's gender characteristics in a more subtle way. Also, she could provide emotional support, being a good friend and listening to Harry's concerns about how others will react.
There could be a mechanic named Tom. He helps Harry keep his helicopter in top shape. Whenever Harry has a problem with the helicopter, Tom is there to fix it.
Snape can be a prominent character. Given his complex relationship with Dumbledore and Harry, his perspective could add a lot to the story. He might have some insights into Dumbledore's actions that Harry doesn't know about, and his reactions to Harry's blaming could vary from being scornful to having a moment of understanding. Luna Lovegood could also be a part of it. Her unique view of the world might make her offer some different viewpoints on the situation between Harry and Dumbledore.
Hermione would likely be a supporting character. She might use her vast knowledge of magic and research skills to help Harry figure out where to go and what to do on his journey to save Voldemort. For example, she could dig up ancient texts about dark magic that might hold the key to Voldemort's salvation.
Another possibility is that the main antagonist could be some sort of ancient evil that is awakened by the events in the fanfic. Dumbledore might have given Snape to Harry because he sensed this new threat. This ancient evil could have powers that are unknown to the modern wizarding world and it would be up to Harry and Snape to figure out how to defeat it.