Some fanfictions might explore a more nuanced past relationship between them. Maybe they had interactions in their younger days that shaped their future enmity. Dumbledore, with his deep understanding of magic and people, could have recognized the darkness growing in Voldemort early on and tried to guide him away from it, but Voldemort's pride and thirst for power made him reject any such guidance.
The relationship between them in fanfiction also affects their portrayals. If the fanfiction focuses on a timeline where Dumbledore and Voldemort were once friends or colleagues, their characters will be different from a story where they were always bitter enemies. Dumbledore might show more regret or a sense of failure if he couldn't prevent Voldemort from turning evil in such a scenario, and Voldemort might have a different level of respect or hatred for Dumbledore depending on their past interactions.
In fanfiction, Dumbledore could have set up a series of trials or quests for Harry before the final battle. These experiences make Harry stronger both magically and mentally. Dumbledore might also have a secret plan that he has been preparing for years. He could be using his own powerful magic in a covert way to weaken Voldemort over time, like gradually breaking down the protection spells around Voldemort's Horcruxes so that Harry can find and destroy them more easily.
Often in fanfiction, Dumbledore helps Harry understand the nature of Voldemort's soul - splitting into Horcruxes. He may guide Harry in the search for these Horcruxes. Dumbledore could also use his own powerful magic to shield Harry from Voldemort's attacks, at least until Harry is fully prepared for the final confrontation.
Perhaps Dumbledore is being forced in this fanfiction. There could be some dark magic or powerful dark wizard threatening Hogwarts or the wizarding world. Dumbledore, in a desperate attempt to save everyone else, gives Harry to Voldemort as a sacrifice. But Harry, being the hero he is, could find a way to turn the situation around and defeat Voldemort from within his lair.
Hermione would be horrified. She would immediately start trying to figure out a way to rescue Harry. She might start researching in the library for some ancient spells or forgotten magic that could help. Ron would be angry at Dumbledore and would probably want to rush off to save Harry without much of a plan at first.
At first, Dumbledore might see Harry as an even greater threat. But as he delves deeper, he could realize that Harry has the potential to be a powerful force for good, despite his dark lineage. So, Dumbledore would start to mentor Harry in a different way, focusing more on helping him come to terms with his identity and use it for positive change.
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.
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.
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.