Romance is also a big theme. You can find fanfictions where characters from Highschool DxD and Fate fall in love. It could be a devil from Highschool DxD falling for a Servant from Fate, or vice versa. This adds an interesting dynamic as they come from different worlds with different values and power structures, and they have to navigate their relationship around all of that.
The most common themes are about power and adaptation. Vegito has such immense power that when he enters the Highschool DxD world, it's a big deal. Writers often focus on how he uses his power in battles against the devils, angels, and other creatures. Also, adaptation is key. How does he adapt to a world full of magic and different species? Does he try to learn their ways or does he force his own Saiyan - Namekian ways onto them? These are the things that make up the core of many fanfictions.
One great Fate fanfiction in the context of a Highschool DxD reader is 'Fate/Highschool DxD Crossover.' It combines the exciting elements of both universes really well. The characters from Fate interact in an interesting way with those from Highschool DxD, and the plot has some great twists.
For Highschool DxD, one of the most popular themes is the exploration of the different factions like devils, angels, and fallen angels. Fanfictions often create new storylines around their relationships and conflicts. In Blue Exorcist, the theme of family is big. Since Rin has a complex family situation, many fanfictions explore that further. Also in Highschool DxD, the theme of forbidden love between different species is quite common, like a devil falling in love with an angel.
One of the popular pairings is Issei and Rin. Issei's bold and perverted (in a comical way) nature from Highschool DxD contrasts well with Rin's strong - willed and magical abilities from Fate Stay Night. Their interactions can create some interesting and humorous situations.
The theme of friendship and camaraderie is often present. In Fate Stay Night, the servants and their masters form bonds, and in Highschool DxD, the main character has his group of friends. Fanfictions might expand on these relationships. Additionally, the idea of facing one's destiny is there. Characters in both are often on a path that they have to follow, and in fanfiction, this can be twisted or explored further. For example, in Fate Stay Night, Shirou's journey to become a hero can be re - imagined, and in Highschool DxD, Issei's growth as a devil can be given new perspectives.
Another popular theme is alternate relationships. The multiverse allows for different pairings between characters that might not be possible in the original story. It gives a fresh perspective on the characters' personalities and how they interact with others.
A common theme is power - up. Rias may gain new powers or improve her existing ones. This could be related to her devil heritage or through some special training or encounter.
In highschool dxd, Issei is extremely popular to write about. His journey from a perverted and weak character to a powerful devil is a great source of inspiration. Rias is also popular as she is a strong and beautiful devil. In fate stay night, Saber is often written about. Her noble and mysterious nature makes her a favorite. Archer is also popular due to his complex personality.
Issei is extremely popular in highschool dxd fanfiction. He's the main character in the original series, and fanfiction writers often like to expand on his adventures, his relationships, and his growth as a character. They might give him new powers or put him in different situations to see how he'll react.
One popular trope is the 'Fish out of Water' scenario. For example, a character from Prototype suddenly finding themselves in the Highschool DxD world full of devils, angels, and other supernatural beings. It creates a lot of comical and interesting situations as they try to adapt.