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.
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.
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.
One great Snape vs Voldemort fanfiction is 'The Dark Mark's Secret'. It delves into Snape's complex relationship with Voldemort. Snape is constantly in a precarious position, having to play the double agent. The story shows how he uses his wits against Voldemort's brute power. Another one is 'Snape's Hidden Alliances'. It focuses on how Snape manages to outmaneuver Voldemort in some crucial situations. The characterizations are really well - done in both of these, making them engaging reads.
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.
For Lily Potter, her husband James Potter might be a likely ally. He would support her in any conflict she has with Dumbledore. Also, her close friends from her school days could side with her. They know her well and would trust her judgment.
The main thing that makes Snape vs Voldemort fanfiction interesting is the contrast between the two characters. Snape is complex, with his own motives and a hidden side. Voldemort is pure evil. So when they interact, there are always mind games and power plays. For example, Snape has to constantly deceive Voldemort, and the tension this creates is exciting.
Well, in a fanfiction, a plotline could be that Percy Jackson is on a quest and somehow crosses paths with Voldemort. Voldemort senses Percy's power and wants to study it. Percy, not knowing much about the wizarding world, initially mistakes Voldemort for just another monster. As they interact more, they realize that their worlds are more intertwined than they thought. Another plot could involve a powerful artifact that both of them are after. The artifact has the power to change the balance of both their worlds. So they are in a race against each other to get it, but along the way, they start to understand each other's motives and might even consider working together to keep the artifact out of the wrong hands.