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.
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.
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.
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.
The Horcrux hunt with Harry, Fleur, and Tonks in fanfiction could be full of adventure. Harry has his own experience and determination. Fleur could add an element of mystery and allure. Tonks, with her unique magic, is quite a valuable asset. They could start by researching the possible locations of Horcruxes in old wizarding libraries. Fleur might be able to charm the librarians into giving them more access. Tonks could change her appearance to look like a different wizarding scholar. Harry would be constantly on the lookout for any signs related to Voldemort. Their journey would be filled with close calls and unexpected allies.
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.
Harry is very determined and has a strong sense of right and wrong. Fleur is proud and confident, often relying on her Veela heritage. Tonks is more laid - back but also very loyal. Their personalities could clash as Fleur's pride might make her not want to take orders from Harry. However, they could also complement each other. Harry's determination could keep them focused on the goal. Fleur's confidence could boost their morale, and Tonks' loyalty would ensure she stays with the group through thick and thin.
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 way is through his connections. Draco has ties to the wizarding world's dark side, which means he could potentially get inside information on the Horcruxes. He might use his family's influence or old Death Eater contacts to dig up secrets.