Another plot could involve Draco actually knowing some secrets about the Horcruxes that he initially doesn't want to share with Harry. But as the danger of Voldemort's power growing becomes more and more real, he realizes he has to help. So, during the hunt, Draco gradually reveals bits of information that he has, which helps Harry in unexpected ways. This could lead to a redemption arc for Draco, as he goes from being on the side of the Death Eaters in a way to actively working against Voldemort by assisting Harry.
Perhaps in the fanfiction, Harry and Draco get separated from the rest of their groups during the Horcrux hunt. They find themselves in a strange and dangerous place, like an ancient, cursed forest. In this isolated situation, they have no choice but to rely on each other. Draco might use his knowledge of Dark Magic (even if it's just to avoid it) and Harry his Gryffindor courage. As they search for a way out and also for the Horcrux they were after, they form an unlikely bond that changes their relationship forever.
Their relationship could start with hostility. Draco has been an enemy to Harry for a long time. But during the Horcrux hunt, they might find common ground. For instance, they both want to stop Voldemort, even if their reasons are different. This common goal could lead to a grudging respect. Harry might see that Draco isn't all bad, and Draco might see Harry as more than just the 'Chosen One' he resented.
One possible plot twist could be that Draco has a hidden agenda all along. Maybe he's actually trying to protect a particular Horcrux for his own reasons. Another could be that he discovers a new Horcrux that no one else knew about, throwing the whole hunt into chaos.
Well, a possible plotline is that they discover a Horcrux in a place that was previously thought to be just a mythical or lost location. Maybe it's hidden in an ancient wizarding ruin that has been long forgotten. As they search for this Horcrux, they encounter all kinds of magical creatures and traps. And also, their relationship might develop romantically during the hunt. They share moments of fear and triumph which bring them closer together.
One way is by using his Slytherin cunning. Draco is known for his intelligence in a sly way. He could outwit the cave's defenses. He might be able to figure out puzzles or riddles that are protecting the Horcrux. His quick thinking could be a great asset.
One possible plotline could be that Harry betrays Draco in a duel. Maybe there was some sort of misunderstanding or a secret deal that Harry made with someone else that required him to turn on Draco during the duel. For example, if there was a dark wizard threatening Harry's friends and the only way to save them was to sacrifice Draco in the duel by using a forbidden spell against him.
In some fanfictions, Draco might use his knowledge of the Dark Arts and Hogwarts' secrets. Since he was raised in a Death Eater family, he could have insights into how Voldemort might have hidden the Horcruxes. For example, he might know of certain places within the school that Voldemort frequented or had an interest in.
Hermione might be one of his allies. Despite their past differences, they could put aside their grudges for the greater good of the Horcrux hunt. Also, Neville could potentially be an ally. Neville has shown great courage and determination, and he might see Draco's newfound willingness to fight against Voldemort as a sign of change.
One possible plotline could be that Harry, through some magical accident or inheritance, comes to own Draco's family manor. Draco is then forced to interact with Harry on Harry's terms to regain his family's pride. Another could be that Harry wins a bet against Draco and the terms of the bet are such that Harry now has ownership over Draco's time, making Draco do tasks for Harry.
One possible plotline could be that Harry has some sort of misunderstanding. Maybe he believes Draco is involved in a dark plot that endangers Hogwarts. So, in a rash moment, he kidnaps Draco to interrogate him. But as he does this, he realizes that Draco might be innocent and then has to deal with the consequences of his actions.
In some fanfictions, Draco might use his knowledge of the Dark Arts. He could have insights into the protections around the cave that Voldemort might have set up, given his family's closeness to the Dark Lord. For example, he might know about certain spells or enchantments that are typical of Dark magic and be able to help the characters navigate through them.