The main themes can vary widely. But often there is the theme of adaptation. Since she is in the High School DxD world, she has to adapt to different situations. Love is also a theme sometimes, whether it's romantic love or love for her comrades. And there's the theme of destiny, like what her true purpose is in this fictional universe which is full of magic, power struggles and various factions.
I've come across 'Lady Phenex's Redemption'. This fanfiction is about her journey to redeem herself for past actions. The story is engaging as it shows her growth and transformation. There's also 'Lady Phenex and the Forbidden Magic' which is filled with mystery and adventure as she gets involved with some powerful and forbidden magic in the High School DxD world.
One common theme is power growth. Since Ravel is from the Phenex family, many fanfictions explore her developing her powers and how she uses them. Another theme is friendship. Ravel often forms new friendships or strengthens existing ones in these stories.
His complex personality. Riser is initially portrayed as a bit of a villain, but he has his own motives and insecurities. Fanfiction writers like to explore these aspects, like his pride which is both his strength and his weakness.
There might be a fanfiction where Riser Phenex discovers a new power within himself. This could lead to a whole new set of adventures as he tries to master it and also deal with the political implications within the Highschool DxD universe, like how other devil families react to his newfound strength.
Another common pairing is Sasuke and Akeno. Sasuke's brooding and powerful nature might attract Akeno, who is also quite powerful and has a bit of a mysterious side herself. They could bond over their shared power levels and a mutual understanding of what it means to be different from the norm in their respective worlds.
Harry Phenex himself is often a very popular character. Since he is the main focus in this fanfiction concept, his unique background as a Phenex in the Highschool DxD setting makes him stand out.
One interesting storyline could be Harry Phenex's adventures in the Highschool DxD world while trying to hide his true identity as a Phenex. He might have to deal with the various factions in the school and their power struggles.
One interesting crossover could be the sharing of power concepts. In Highschool DxD, the Phenex family has unique powers like immortality and fire abilities. In Naruto, there are also various powerful jutsus. A crossover could explore a character from Naruto learning about the Phenex powers and incorporating them into his own fighting style. For example, Sasuke might be intrigued by the Phenex's fire powers and try to combine it with his own fire - based jutsus like the Great Fireball Technique to create a more powerful attack.
The key elements in a Highschool DxD Phenex self - insert fanfiction include the Phenex clan's characteristics, of course. Their abilities, traditions, and status in the devil society. Then, the self - inserted character's journey. How they adapt to the Highschool DxD environment, which is full of different races like devils, angels, and fallen angels. Also, the conflicts. It could be internal, like the character struggling with their new identity, or external, such as facing a threat from another group.