First, you need to establish the relationship between the Hetalia character and the reader. Make it seem loving and believable at the start. Then, introduce the idea of cheating in a way that makes sense within the context. Maybe the Hetalia character has some internal struggles or outside temptations.
Another plot could be the reader character trying to get revenge on Sasuke. This could range from small acts of spite to more elaborate schemes. However, revenge might not always be the best option as it could lead to more problems in the long run.
In some yandere cheater x reader stories, the reader might initially be unaware of the yandere's cheating ways. For example, the yandere could be very charming and deceive the reader into believing they are single. But as the story progresses, the reader discovers the truth and has to deal with the yandere's overbearing and sometimes dangerous behavior.
One common theme could be betrayal. Since Hugh is a cheater in this scenario, the reader character might experience a sense of betrayal, which can lead to a lot of emotional turmoil and drama in the story.
One common theme could be the male reader's discovery of the cheating, which often leads to a lot of emotional turmoil. For example, he might feel betrayed and angry.
In other cases, the yandere might reform their ways. After some events that make them see the error of their cheating ways, they might try to be faithful to the reader. This could involve a lot of groveling and promises to change. However, the reader may or may not believe them, and the relationship remains on shaky ground.
One common theme is betrayal. Since it's about a cheater male, the sense of being betrayed by someone you love or trust is a big part of it. Another theme is heartbreak. The male reader usually experiences a great deal of emotional pain.
You can try searching on some popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of various fanfictions, including 'cheater link x reader' ones. Just use the search bar and type in the relevant keywords.
One popular 'cheater link x reader fanfiction' might be those where the reader discovers Link's cheating ways and has to deal with the heartbreak. For example, in some stories, the reader could be a character in the Zelda universe who has a relationship with Link, and then finds out he has been unfaithful with another character like Zelda herself. It often involves a lot of drama and the reader character's journey to overcome the betrayal.