Love and romance are very prevalent. The relationship between Zeldris and the reader character is usually explored in a romantic way, with all the emotions like love, jealousy, and passion.
Another theme could be power and growth. Zeldris has his own power, and in some stories, the reader character might grow in power alongside him or be inspired by him. This can add an interesting dynamic to the relationship in the story. In addition, there may be themes related to loyalty and sacrifice, as these are often important aspects in the original source material that Zeldris comes from. The stories may explore how the relationship between Zeldris and the reader is tested by situations that require loyalty or sacrifice.
You can try some fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. These platforms usually have a wide variety of zeldris x reader stories.
The theme of redemption can be found in some Levi x Reader stories as well. Maybe the reader has a past they are not proud of, and Levi helps them find a way to redeem themselves. Or it could be the other way around, with Levi having some internal struggle that the reader aids him in overcoming. These themes add depth to the relationship between the two characters in the stories.
Adventure is a common theme. Hiccup is known for his adventures, and when paired with the reader, they often go on quests, like exploring uncharted islands or finding lost dragon species.
One common theme is the love for volleyball. In these stories, whether it's the reader or the Haikyuu characters, their passion for the sport is always evident. Also, teamwork is crucial. The reader usually has to work together with the team members to achieve their goals, like winning a match or improving the team's overall performance. And there is often a bit of a romantic undertone as well, as the reader may develop feelings for one of the characters.
Adventure is a very common theme. The reader usually embarks on an exciting journey, whether it's through a magical land or in outer space. Another theme is self - discovery. As the reader interacts with the epic world and its characters, they often learn more about themselves.
One common theme is cultural exchange. Since the Hetalia characters represent different countries, there's often a focus on sharing cultural aspects between the character and the reader. Another theme is adventure. The reader might go on an exciting journey with a Hetalia character.
Love and acceptance are common themes. Since mermen are from a different species, there's often a plot about the reader accepting the merman's differences and falling in love despite them.
One common theme is self - discovery. As the reader interacts with Dream in the story, they often learn more about themselves. Another theme is the power of the imagination, since dreams are all about the mind's creativity.
Powerlessness is also a key theme. In stories with horror characters like the Annabelle doll. The reader often feels powerless against the supernatural forces at play. The horror character has abilities or powers that the reader doesn't have, and they have to find a way to either overcome that or at least survive in the face of such overwhelming power.
One common theme is friendship. The human Pokemon and the reader often form a strong bond of friendship. Another is adventure, as they usually go on quests or explore new places together.