Well, in such fanfictions, often there's a theme of emotional turmoil. The male reader is likely to go through a range of emotions like anger, sadness, and confusion. Also, there might be a theme of self - discovery for the male reader. He may learn more about himself and his boundaries as a result of the cheating situation. And sometimes, there could be a theme of revenge where the male reader tries to get back at the cheater in some way.
Well, one place to start looking could be fanfiction.net. There are often a variety of RWBY fanfictions there. However, I'm not sure specifically which ones are about a 'cheater' scenario with a male reader. You might have to do some searching through the RWBY section and look at the summaries to find ones that match.
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.
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.
Create a believable setting. It could be a college campus or a small town. This setting can influence how the characters interact. If it's a small town, rumors might spread faster about Erin's cheating. Also, build up the conflict. Don't make the discovery of cheating too sudden. Let there be some hints and suspicions first.
One common theme is the process of the heartbroken male reader's healing. It often shows how he overcomes the pain caused by the cheater male. Another theme could be revenge, where the reader might take actions to make the cheater feel the same pain. Also, self - discovery is a theme, as the reader might find out new things about himself during this difficult time.
It could end with the heartbroken male reader finding new love. He moves on from the cheater male and meets someone who truly values him. This new relationship helps him heal completely.
One popular 'cheater male x male reader fanfiction' is 'Broken Promises'. In this story, the male reader discovers his partner's cheating ways and has to deal with the heartbreak and betrayal. It's a very emotional read.
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.