One idea could be Subaru having to face his old fears in his world that he thought he had overcome in the other world. For example, a bully from his school days who now has some sort of power. Another could be that the knowledge he gained in the other world gives him an edge in his own world, like being able to solve complex problems in school or help his family business. Maybe he also has to hide the fact that he's been to another world from his friends and family because they wouldn't believe him and might think he's crazy.
A possible plot is that Subaru discovers that there are some hidden connections between his world and the other world he visited. There could be a secret organization in his world that is aware of the existence of other worlds and they start to target Subaru because of his knowledge. He has to figure out how to deal with them while also trying to readjust to his normal life. He may also find that some of the people he met in the other world have doppelgangers or counterparts in his own world with strange similarities and differences.
Perhaps in the fanfiction, Subaru returns to his world but with a part of the other world's magic still in him. This magic could start causing strange things to happen around him. He might accidentally use it in front of his friends and then have to cover it up. Or, he could use it to help someone in need, like saving a neighbor from a fire by creating a magical shield. Also, his experiences in the other world could have changed his personality, making it difficult for him to fit back into his old social circles at first.
A new character could be a scientist who is intrigued by Subaru's sudden change in behavior and knowledge. This scientist might suspect that there's something otherworldly about Subaru and try to study him. Another could be a mysterious stranger who seems to know more about Subaru's time in the other world than he should. Maybe this stranger has also had some experiences with other worlds or has some special powers related to it.
One interesting 'Re: Zero Evil Subaru' fanfiction could be one where Subaru's evil side is a result of his repeated deaths and the corruption of his mind. In it, he might start using his knowledge of the loops in very unethical ways, like manipulating others to achieve his own goals without regard for their well - being.
One interesting plot could be Subaru and Ram going on a journey together to find a rare magical item. Along the way, they face various challenges and their relationship deepens as they rely on each other for survival.
In some fanfictions, there might be a plot where Felix and Subaru team up to save a character from a great danger. They could be facing a powerful enemy that neither could defeat alone, but together, they manage to outwit the foe using their combined skills and knowledge of the Re:Zero universe. Felix's magic and Subaru's Return by Death ability could play crucial roles in this adventure.
Another idea is to write a fanfiction where the roles are reversed. Make Emilia the one with a special power and Subaru as the more 'normal' character who has to support her. It could be fascinating to see how Emilia would handle the burden of a power like Return by Death and how Subaru would adapt to being in a more supportive role.
Another plot could involve the Pleiades from Overlord. They end up in Re:Zero and are mistaken for some sort of magical spirits. Subaru tries to enlist their help in his quest to save Emilia. The Pleiades, with their combat skills and unique abilities, could change the power dynamics in the Re:Zero world and also introduce new technologies and magic concepts from Overlord.
Yes, there are many. Some might explore their relationship in different scenarios within the world of Re:Zero. For example, a story could be about Elsa and Subaru teaming up to face a new and powerful enemy in the kingdom.
Another possibility is a crossover with 'My Hero Academia'. Subaru could be a student at U.A. High School. His Return by Death power would be like a quirk. He might struggle at first to control it and fit in, but eventually use it to save his classmates from villains, in a way that surprises everyone with his growth and resourcefulness.
You could create an OC who is a master of illusions. This character could use their powers to deceive both enemies and allies, and their backstory could involve being an outcast from their own illusionist clan because they refused to use their powers for evil. In the Re:Zero world, they could help Subaru by creating diversions during battles or by using illusions to uncover hidden truths.
One common theme is friendship. Fanfictions often explore how Subaru forms deep friendships with the female characters. Another theme is growth. It shows how Subaru and the female characters grow together through their experiences.