One common theme is the idea of forbidden love. Since Hades is the god of the underworld, his relationship with a mortal (the reader in these fanfics) is often seen as something that goes against the norms. Another theme is transformation. The reader usually has to adapt to the strange and often dark world of the underworld, which changes them in many ways.
Love conquering all is perhaps the most prevalent theme. Despite all the differences between the reader and Hades, such as their different natures (mortal vs. god) and the gloomy nature of the underworld, their love manages to thrive. It gives the message that love can bridge any gap and overcome any obstacle, no matter how daunting.
One popular 'reader marries Hades' fanfic is 'Underworld Love'. It has a really interesting plot where the reader character is brought to the underworld and gradually falls in love with Hades. The descriptions of the underworld are very vivid.
Another theme is rebirth and transformation. Persephone's journey between the upper world and the underworld can be seen as a form of transformation, and her love with Hades is often part of that process. It's like she discovers a new side of herself through their relationship. In many fanfics, this is explored in different ways, like how she becomes more confident or assertive because of her love for Hades.
Love against all odds is also a major theme. Given their different backgrounds and the fact that in the original lore they are from very different social groups, their love has to overcome many prejudices and challenges in these fanfics. For example, in some fanfics, Hermione has to deal with the pure - blood supremacist views that Abraxas may have been associated with in his time, and Abraxas has to understand Hermione's more egalitarian views.
One common theme is their rivalry. Apollo, the bright and positive god, and Hades, the lord of the underworld, can have an interesting dynamic of light vs. dark, good vs. evil (in a very loose sense in Greek mythology). Another theme could be their cooperation. They might work together to solve a problem that affects both the upper world and the underworld, like a threat to the stability of the cosmos.
One common theme could be the anticipation and excitement of new life. It might also involve the relationship between Sam and the reader, how they deal with the changes that come with pregnancy.
A common theme is adventure. Since Aqualad is often involved in underwater quests and superhero battles, the reader in the fanfics usually joins him in these exciting adventures. Another theme is friendship or romance. The relationship between the reader and Aqualad can range from being really good friends to falling in love. For example, in many fanfics, they start as comrades and then feelings develop over time.
Often, there are themes related to self - discovery. The reader character might learn about themselves through their interactions with Lucy. For example, they could discover new powers or aspects of their personality. Also, themes of loyalty are quite common. Whether it's the reader being loyal to Lucy or vice versa, it adds an interesting dynamic to the story. In some fanfics, there are also themes of family, where Lucy and the reader become like family to each other, providing a sense of home and belonging in the fictional world.
One common theme is adventure. Since 'killua' is from the 'Hunter x Hunter' universe which is full of exciting and dangerous adventures, many fanfics will have the reader character and Killua going on quests together. Another theme is friendship and trust building. Killua has a complex personality, and fanfics often explore how he gradually opens up to the reader character, forming a deep bond of trust.
One common theme is mentorship. Since Celestia is a wise and powerful figure, she often takes on the role of mentoring the reader in fanfics. Another theme is adventure. The reader usually goes on some sort of adventure with Celestia, whether it's exploring Equestria or a more magical realm. Friendship is also a big theme, as the relationship between the reader and Celestia develops into a strong friendship in many of these fanfics.
One common theme is redemption. Loki is often a complex character who has made mistakes in the past, and in fanfics, the Reader can play a role in his path to redemption. Another theme is love against all odds, as Loki's status as a trickster and sometimes an outcast can make a relationship with a normal Reader seem unlikely but all the more interesting.