The antagonists could be some shadowy figures from the magical world who are afraid of the potential power couple that Harry and Susan could become. If they get married and combine their resources and powers, it could disrupt the existing power balance in the magical realm. So, these figures might try to create misunderstandings between Harry and Susan, or even directly attack them to stop the marriage contract from being fulfilled. There could also be a magical curse or entity that is somehow tied to the marriage contract and causes problems for them, acting as an antagonist in the story.
Well, it could be an old enemy of Harry's who doesn't want to see him happy in this new relationship. Maybe a dark wizard who wants to disrupt the peace that the marriage might bring. Another option is a jealous rival of Susan's. Someone who had feelings for her before and now is enraged that she's in a marriage contract with Harry.
In this kind of fanfiction, the antagonists could be members of their own families. For instance, if there is a relative in Harry's family who has a different vision for his future and doesn't approve of the marriage contract with Susan. They could try to undermine the relationship. Similarly, in Susan's family, there might be someone who has their own ulterior motives and sees the marriage as a threat to their own plans. Additionally, there could be a powerful magical organization that has rules against such unorthodox marriage contracts and they try to stop Harry and Susan from fulfilling it.
Voldemort could be an antagonist. Even if Tom is a character in the marriage contract, Voldemort's ideology and his followers might oppose the union as it could potentially change the power dynamics in the wizarding world that they are trying to control.
In a 'Harry and Susan Marriage Contract Fanfiction', a possible plot point is how they deal with their own personal goals. Harry might have his own heroic or magical pursuits, and Susan could have her own dreams. The marriage contract could either support or conflict with these goals. And then there's the aspect of friendship. How do their existing friends react to this sudden marriage contract? Do some friends try to break them apart, or do they help them make the best of it? That can add a whole new layer to the story.
Another possible antagonist could be a jealous student. Maybe someone who was not chosen for the Triwizard Tournament and feels left out. They might try to sabotage Harry and Susan's chances in the tournament out of envy.
It could be someone who wants to break their marriage contract for personal gain. Maybe a jealous ex - lover of Harry who doesn't want to see him with Tracey. This person could try to create misunderstandings between them and spread false rumors.
One possible plot could be that after the war, Harry and Susan cross paths while helping to rebuild Hogwarts. They start to work together on a project, like restoring the Greenhouses. As they spend more time together, they realize they have a deep connection and fall in love. Eventually, they get married and combine their families' traditions at the wedding, with Harry's wizarding world fame and Susan's Bones family heritage.
The pure - blood family that arranged the marriage contract could be the main antagonist. They might have a lot to lose in terms of status, power, or wealth if the contract is broken. So, they would do everything in their power to force Harry to uphold it, even using dark magic or underhanded tactics.
One possible plot could be that Harry and Tracey enter into a marriage contract for convenience. Maybe Harry needs to inherit something and being married to Tracey is a condition. They start off as strangers but gradually get to know each other through the requirements of the contract. They face challenges from their families who initially don't understand their motives.
Well, it could be a very interesting union. Harry Potter, being the hero he is, and Susan Bones, a smart and brave witch. Their marriage might bring together two different yet powerful magical lineages. Maybe they'd face challenges from the wizarding world still recovering from the war, but their love would be a source of hope and stability.
Hermione is a very common character paired with Harry in such fanfictions. Given their strong friendship in the original series, it's not a stretch to imagine a marriage contract situation between them. Their shared intellectual pursuits and adventures could form a strong basis for a relationship under a marriage contract.
They could be two ordinary friends who have just decided that they need a change in their lives. Maybe they've grown up together in a small English town and now want to see what the world outside England has to offer.