Dumbledore might give Harry advanced spellcasting training. He could teach Harry some of the more complex and ancient spells that are not commonly taught at Hogwarts.
It might make Harry more confident. With Dumbledore's training, Harry gains more knowledge and skills, which gives him the confidence to face challenges.
The relationship between Harry and Dumbledore in this fanfiction can be really fascinating. Dumbledore is a very powerful and wise wizard, and his training of Harry might involve not only magic but also life lessons. They could have some deep conversations that shape Harry's view of the wizarding world and his role in it.
They probably have a very close relationship. Dumbledore would be like a father figure to Harry, guiding him through the wizarding world.
Another possible antagonist could be a group within the wizarding world who doesn't want Dumbledore's plans to be exposed. They might have been benefiting from Dumbledore's secrecy or they believe that his plans, although seemingly unethical to Harry, were necessary for the greater good. They would then try to stop Harry from revealing the truth.
In such a fanfiction, perhaps a major plot point is how Dumbledore trains Harry to face a new and greater threat. It could be a dark wizard more powerful than Voldemort, and Harry has to learn a whole new set of strategies and magic under Dumbledore's guidance. Harry might also develop a deeper understanding of the wizarding world's history and secrets through Dumbledore's teachings, which could be a key part of the plot.
Well, in some fanfictions, Harry might attack Dumbledore if he discovers that Dumbledore has been using him as a pawn in a bigger game all along. This feeling of betrayal could drive him to lash out. Also, if there is a prophecy that he misinterprets to mean Dumbledore is a threat to his future or the future of his friends, that could be a motive.
Perhaps Dumbledore is shown to have a secret alliance with Voldemort in the fanfiction. Harry stumbles upon evidence of this and is filled with rage and hatred. Or maybe Dumbledore had been manipulating Harry's emotions and actions from the very beginning to fit his own plans for the wizarding world, and once Harry realizes this, his hatred for Dumbledore grows. It could also be that Dumbledore has been hiding a powerful magical artifact that could have saved many lives but didn't, and Harry blames him for it.
Well, it might be a fanfic where a new dark wizard emerges with a unique power to manipulate memories, and they target Dumbledore's memories of Harry Potter. Or it could be a story where Dumbledore gets caught in a complex web of magical experiments gone awry, leading to this memory loss.
Perhaps there are safety concerns for the students that Dumbledore is not addressing adequately. Harry, having a strong sense of responsibility for his fellow students, may feel that the only way to ensure their safety is to remove Dumbledore from his position. He might believe that Dumbledore's actions or inactions are putting the students at risk, and thus he takes matters into his own hands.
Well, it could be that Harry has a different view on how to handle the war against Voldemort. Dumbledore is often very strategic and sometimes keeps things from Harry for what he believes is the greater good. But Harry might think that full transparency is necessary. Also, Harry may have formed new alliances or friendships that give him different perspectives, and these new ideas conflict with Dumbledore's. He might feel that Dumbledore's approach is too old - fashioned or risky in a new context.