Some of the more fanatical Death Eaters could also be antagonists. They might not trust Harry even with the Dark Mark and constantly try to undermine him or expose him as a spy. They could set traps for him or try to force him into more dangerous and evil tasks than Voldemort originally intended.
There could also be a new character created in the fanfiction who is in charge of making sure Harry obeys the commands associated with the Dark Mark. This character could be a powerful dark wizard or witch who has a personal vendetta against Harry and uses the situation of the Dark Mark to torture and manipulate him.
Obviously, Voldemort would be a main antagonist. Since it's the Dark Mark of Voldemort that Harry is forced to take, Voldemort would be pulling the strings, trying to make Harry do his bidding.
Another aspect could be his internal struggle. Harry might have nightmares and visions related to the Dark Mark. He'd be constantly on edge, worried about being controlled by Voldemort. His relationships with his friends, like Ron and Hermione, would be severely tested. Hermione might try to research ways to remove the Dark Mark while Ron would be more impulsive, wanting to directly confront the situation, which could lead to conflicts among them.
One consequence could be that the Death Eaters no longer sense his connection to Voldemort. This gives Harry an advantage in his fight against them as they can't track him through the Mark.
In some fanfictions, Harry might use powerful ancient magic that he discovers in a hidden library or from an old wizard. This magic could be specifically designed to counter the dark magic of the Dark Mark.
Maybe it's a jealous classmate. Someone who is envious of Harry's fame and power. This person discovers a dark spell that forces Harry to be a girl, thinking that it will make Harry lose his status among his friends and in the wizarding community at large. They then watch from the sidelines as Harry struggles with this new identity.
Dumbledore could be seen as the main antagonist from Harry's new perspective on the dark side. Dumbledore represents all that is good in the wizarding world, and if Harry has turned to the dark, Dumbledore would be the one trying to stop him, to bring him back to the light. So in this context, Dumbledore becomes the opposition that Harry has to face.
Maybe it's a new character, a powerful dark wizard who has a vendetta against Dumbledore. By forcing Harry into the tournament, they hope to draw Dumbledore out and make him vulnerable. This new antagonist could have some dark magic at their disposal that allows them to control the Goblet's selection process.
Some of the die - hard Death Eaters could also be antagonists. They might not trust Harry even with the Dark Mark and try to undermine him or expose his true intentions if they suspect he's not truly on their side. Bellatrix Lestrange, for example, is so fanatical about Voldemort that she would be a likely candidate to oppose Harry even if he seems to be part of the 'dark' group.
In addition to the above, other characters from the wizarding world could be involved. For example, the Order of the Phoenix members. They might be suspicious of Harry at first but then come to his aid. And there could be some new characters created specifically for the fanfiction. Maybe a mysterious old wizard who has knowledge about the Dark Mark that no one else does and helps Harry on his journey to rid himself of it or at least understand it better.
It could be Dumbledore. In a twisted take on the story, if Harry joins the dark side, Dumbledore would be the one trying to stop him. Dumbledore has always been the guiding force for the good side, and Harry's defection would be a huge blow. Dumbledore would then have to face the difficult decision of either trying to save Harry or stopping him by any means necessary, making him the antagonist from Harry's new 'dark' perspective.
It could be a new dark wizard who has been secretly building his power. Maybe this wizard uses the allure of the cane and the idea of a new kind of dark magic to draw Harry in. He might be a former follower of Voldemort who has his own plans for domination and sees Harry as a useful tool.