One possible plot could be that in an alternate universe where wizarding traditions are very different, Draco has committed some minor offense in Hogwarts. As a form of punishment, he is made to be led around by a leash by the protagonist, who could be a Gryffindor student. This would lead to a lot of power - play and interesting character dynamics as Draco, who is used to being in a position of privilege, has to endure this humiliation. Along the way, they might start to understand each other better and form an unlikely friendship.
Perhaps it's a part of some sort of new magical sport or activity in the wizarding world. Draco is chosen as a participant against his will and has to be led around on a leash by another wizard or witch. At first, he is extremely resistant, but as he gets into the rhythm of the activity, he begins to see the value in it. The fanfiction could focus on his transformation from a reluctant participant to someone who actually enjoys the experience and how it changes his relationship with the person leading him.
It could be Hermione. Given her strong - willed nature and her often - clashing relationship with Draco, she might be the one to put him in such a situation. Maybe she discovers a new spell or magic item that allows her to do this as a way to keep Draco in line during a particularly tense time at Hogwarts, like when there are threats from dark forces and she doesn't trust Draco not to side with the wrong people.
One idea could be a story where Draco has lost his memory and the character leading him around on a leash is actually trying to protect him from the dark forces that are after him. They use the leash as a way to keep him close in this dangerous situation.
One possible plot could be that Draco is in a situation where he is being controlled by a leash for some magical or political reason. Maybe he's being paraded as a symbol of a new order in the wizarding world. His former Slytherin pride is put to the test as he has to endure this public display.
Well, in 'halfbreed draco leash fanfiction', there could be themes related to acceptance. The halfbreed might struggle to be accepted in a world where Draco (who might be from a more 'normal' or dominant group) exists. The leash could also represent a connection that is both physical and emotional. It could be a source of comfort or a source of conflict. Another theme might be transformation. The halfbreed could be in a process of changing, and Draco's presence (along with the leash as a symbol) could either aid or hinder that transformation. There could also be themes of mystery, especially if the origin of the halfbreed or the purpose of the leash is not immediately clear.
The plot could involve a character who discovers Draco, a mysterious and powerful creature, and decides to keep it on a leash for safety reasons. But Draco doesn't like being restrained and tries to break free. This leads to a series of adventures as the character tries to keep Draco under control while also uncovering the secrets of Draco's origin.
One possible fanfiction could be about Draco using a harness leash in a magical creature training scenario. Maybe he's trying to tame a rare and unruly beast, and the harness leash is a special magical tool. He has to use his Slytherin cunning to figure out how to work with the creature while using the leash effectively.
In 'leash on draco fanfiction', it could center around a power dynamic. Draco is often seen as a proud and somewhat haughty character. So having him on a leash could represent a shift in power. For example, another character might be in charge of him, and this could lead to all sorts of interesting interactions, like the character using the leash as a means to make Draco do things he wouldn't normally do, or it could be a part of a redemption arc where Draco has to submit to certain conditions to grow as a person in the fanfiction world.
One possible fanfiction could be about a magical creature Draco that is tamed and put on a leash by a powerful wizard. The wizard might be using Draco for some sort of mission, like exploring a dangerous enchanted forest. Draco could be a dragon - like creature with its own will, and throughout the story, it struggles against the leash both physically and emotionally as it tries to assert its independence.
Draco and a harness leash could be part of a story where he's in a magical circus. He's in charge of some magical animals and the harness leash is what keeps them in line. But Draco might start to have second thoughts about keeping these animals in captivity, and the harness leash becomes a symbol of his internal struggle between duty and compassion.
I'm not sure specifically which ones are the most popular, but there are likely some out there. Fanfiction websites are filled with all sorts of creative takes on characters, and a concept like 'leash on draco' is bound to attract some attention. You could start by looking on major Harry Potter fanfiction platforms and search for stories with that title or related tags.