For Harry, his friends like Ron and Hermione are likely to be his allies. They've always been by his side through thick and thin in the original series, so it's natural for them to support Harry in a confrontation with Dumbledore. Neville could also be an ally, given his growth and courage shown in the books.
Hermione is often a very popular character. She's known for her intelligence and resourcefulness, and in fanfiction, she might come up with new spells or strategies to help Harry in his fight against Voldemort.
One interesting plotline could be a magical duel between them where they both use unique spells not seen in the original books. Maybe Dumbledore is under the influence of a dark spell, and Harry has to fight him to break the curse.
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.
One interesting plotline could be an alternate universe where Harry and Voldemort have to team up against a greater evil. This flips the traditional dynamic on its head.
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.
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.
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.
Well, Harry might outsmart Dumbledore by using his unique understanding of the magical world in ways Dumbledore doesn't expect. For example, Harry has a more direct experience with some dark magic items due to his encounters with Voldemort. He could use this knowledge to make decisions that seem counter - intuitive to Dumbledore but are actually more effective in a particular situation.