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.
One possible plot could be that Harry discovers some hidden motives behind Dumbledore's actions. Maybe he finds out that Dumbledore was using him in a way he didn't expect, like in a grand scheme against Voldemort that put Harry in more danger than necessary. So, Harry decides to expose these plans to the wizarding world, which would cause a huge upheaval in the relationships between characters.
Perhaps Harry misinterprets Dumbledore's cryptic messages. Dumbledore often gives rather mysterious instructions, and if Harry takes them the wrong way, it can lead to him messing up the plans. Like he might think Dumbledore wants him to do one thing when in fact it's the opposite, and so he goes ahead and does the wrong thing, thus ruining the plan.
In some fanfictions, Harry might derail Dumbledore's plans by having different priorities. For example, if Dumbledore's plan is centered around a long - term strategy against Voldemort that involves sacrifice and patience, Harry could derail it by acting impulsively. He might decide to take on Voldemort directly, earlier than Dumbledore intended, believing he can end the threat immediately. This could be due to his own sense of justice and the desire to protect his friends more quickly.
The consequences might include a change in the outcome of the fight against Voldemort. If Dumbledore's plan was a sure - fire way to defeat Voldemort in the long run, Harry derailing it could lead to either a more difficult victory or even a defeat. Also, Harry himself could face personal consequences. He might lose the trust of his friends who believed in Dumbledore's wisdom. His reputation in the wizarding world could be tarnished as people would think he was acting recklessly.
One consequence could be that other characters are put at risk. If Dumbledore had a plan to protect everyone and Harry messes it up, those protections might fail. For example, if Dumbledore planned a safe passage for a group and Harry's actions cause the enemies to discover it, the group could be in danger.
It could also be that Harry has a different vision for how to deal with the main threat in the wizarding world. Dumbledore's plans might seem too slow - paced or too conservative to Harry. He may believe that his own more direct and bold approach would be more effective, and thus decides to destroy Dumbledore's plans to implement his own.
In the wider wizarding world, it could make Harry a controversial figure. Some might see him as a rebel who is undermining the great Albus Dumbledore. Others, especially those who have had doubts about Dumbledore's secrecy, might start to view Harry as a brave truth - seeker. This would change how various wizarding families and factions interact with Harry, either distancing themselves or becoming more interested in his cause.
Dumbledore, as Harry's guardian in the fanfiction, would probably use his influence in the wizarding world. He could make sure that Harry is taught by the best teachers at Hogwarts. He'd also be there to offer advice and guidance, especially when Harry faces challenges like dealing with Voldemort or other dark forces. For example, he might teach Harry some advanced defensive magic privately to better protect himself.
In some fanfictions, Harry might be depicted as Dumbledore's pet in a metaphorical sense. It could be that Dumbledore is overly protective or manipulative in the story, making Harry seem like a 'pet' under his control. But this is not a common or canonical view, just a creative interpretation in certain fan - created works.