First, think about your OC's personality. Are they brave like Eren or more cautious? Then, decide on their backstory. Maybe they come from a family of outcasts within the walls. Next, consider how they will interact with Eren. Will they be friends, rivals, or something else? After that, plan out the plot. It could be about a new threat to the walls that only your OC and Eren can face together. And don't forget to give your OC unique abilities or traits that will make them stand out in the story.
A great Eren Yeager OC fanfiction I read was about an OC who was a half - Titan. This OC had to hide their true nature from everyone, including Eren at first. But as the story progresses, they reveal their identity to Eren and together they try to find a way to end the Titan curse once and for all. It's full of action, drama, and explores the complex relationship between humans and Titans in a new way.
The idea of Eren Yeager in a harem scenario is interesting because it takes a character from a serious and often dark story and puts him in a more light - hearted, relationship - focused situation. It allows for different character interactions that we don't see in the original work.
There are various 'Eren Yeager x Harem' fanfictions. Some stories could be about Eren being in a harem situation where he has to balance his duties and his relationships. Maybe in one, he is training to fight Titans while also navigating his feelings for multiple girls. These fanfictions often play with the existing character dynamics and create new scenarios for the characters to interact in.
In a post - apocalyptic fictional story, there could be a group of survivors who get stomach rumbling and diarrhea after eating some contaminated food they found. This would be a big problem as they are already in a difficult situation. Some of them might think it's a new virus or something, while others would be more concerned about finding clean water and proper medicine. It would create a lot of internal conflict within the group and also make them more vulnerable to outside threats.
Well, in a fictional adventure story, a traveler in a strange land could get stomach rumbling and diarrhea from eating an unknown fruit. This would then lead to a series of events like seeking help from the local tribes, who may or may not be friendly, and it could be a turning point in the story, either making new allies or facing new enemies.