Well, there are quite a few. For example, alternate universe fanfictions are really popular. In these, the characters are placed in a completely different setting, like a modern - day Earth without magic but they still have their powers. Then there are backstory - focused fanfictions, where the author delves deeper into the characters' pasts, like what Issei was really like before he became a devil. Also, there are crossovers with other anime or manga series, which can create really interesting and unique stories.
I'm not sure if there is a specific fanfiction with that exact plot. Fanfiction can cover a wide range of themes and scenarios in the High School DxD universe, but this particular one might be a bit more niche. You could try searching on popular fanfiction websites like FanFiction.net or Archive of Our Own to see if it exists.
Some common themes are love and loyalty. Since it's a fanfiction about Issei and Yubelluna, their relationship could be explored in terms of how deep their love grows and how loyal they are to each other. Also, the theme of redemption can be present. Maybe Yubelluna has a past she wants to atone for and Issei helps her through it. Additionally, the theme of friendship within the context of their relationship can be explored, like how they support their friends while also focusing on their own relationship.
One popular theme could be Issei's growth in power with the influence of sunshine. Maybe in the fanfiction, the sunshine represents some kind of special energy that enhances his abilities. Another theme might be his relationships with other characters in the context of this sunshine element. For example, how his interactions with Rias or other members of the Occult Research Club change because of it.
There are many popular High School DxD Issei fanfictions. One type often focuses on his growth as a character. For example, some fanfictions explore his relationships with other characters in more depth than the original work. They might show how he further develops his powers and how his relationships with the various devils, angels, and other supernatural beings evolve. Another popular aspect in fanfictions is alternative storylines, like what would happen if he made different choices during certain key events in the story.
One idea could be exploring the psychological impact on Issei. He might feel frustrated or confused about this new situation. It could also lead to interesting relationship dynamics, like how his partners react and support him through this.
Adventure is sometimes a theme. Even during a honeymoon, the story might take Issei and his partner on some exciting adventures, whether it's exploring a new magical place or facing some challenges together, which adds an extra layer of excitement to the fanfiction.
Well, Issei in High School DxD fanfiction is significant for multiple reasons. He is the protagonist in the original series, so in fanfiction, he serves as the anchor for many alternate storylines. Writers can explore his backstory more deeply, like his family life before becoming a devil. His interactions with different factions in the High School DxD universe, such as the devils, angels, and fallen angels, offer a wealth of possibilities for fanfiction. Fanfiction can also focus on his training and how he hones his skills, which is an exciting aspect for many fans.
I'm not sure specifically as there could be many different interpretations of such a fanfiction. It might be about Issei from the 'High School DxD' series achieving a certain goal or having a particular experience, but without reading it, it's hard to say for sure.
One popular theme is the combination of different magic systems within the High School DxD universe. For example, mixing the traditional magic of the devils with magecraft from other sources.
The main reason is the unique dynamic between Issei and Ophis. Issei is a perverted yet determined character, while Ophis is this all - powerful, mysterious being. Their interactions, whether it's Issei's attempts to understand Ophis or Ophis showing a new side of herself to Issei, are very interesting.