A mysterious transfer student could be added. This student could have a hidden power or a special connection to the harem queen's past. For example, she might be from a long - lost clan that has some sort of prophecy related to the harem.
A foreign exchange student could be a great addition. She could come from a far - off land with unique powers and a different culture. This would add an interesting dynamic to the harem as she tries to fit in. Another potential character could be a mysterious girl who is actually a powerful supernatural being in disguise. She could initially be an antagonist but then end up falling for the main character. Also, a shy girl who is extremely talented in magic but lacks confidence could be added. The main character could help her gain confidence and she would then become part of the harem.
A new magical teacher could be added. This teacher could have a special connection to Kuroka, perhaps knowing about her past in ways others don't. They could guide her in her magical studies and also add some mystery to the story.
A human student with latent supernatural powers could be added. This character could befriend Gasper and his harem, and their discovery of their powers could be a major plot point. They might also cause some jealousy within the harem as they get closer to Gasper.
A heroic knight from a medieval - themed anime. He could be transported to the Highschool DxD world and, due to his chivalrous nature, attract the female characters. He might initially be at odds with the devils but eventually form a bond with the main character and become part of the harem as he adapts to the new world.
In Highschool DxD, Asia Argento is a sweet and gentle character. Given her kind heart, she could very well be part of the harem in an OC fanfiction. She might be attracted to an OC who shows her kindness and protects her. Xenovia could also be a part of it. She's strong - willed and dedicated to her beliefs. An OC with a different perspective on things might catch her attention. And let's not forget Irina Shidou. Her energetic and somewhat naive nature could lead her to be interested in an OC with an exciting backstory.
There could be Issei from the 'Highschool DxD' series. He might be involved in the fanfiction, perhaps having a different relationship with Ophis compared to the original series.
A futa version of Rias could be an interesting main character. She could have all of Rias's charm and leadership skills, but with the added element of her futa nature, which could lead to different power manifestations and relationship developments, especially with Issei.
Obviously, the third devil would be a main character. But also, characters like Issei could still play a major role. They might interact with the third devil in various ways, such as training together or getting into conflicts. Another potential main character could be a new character that you introduce specifically for this fanfiction. Maybe a mysterious transfer student who has some connection to the third devil's past or powers.
A character like Geralt from The Witcher could be an interesting crossover. He has his own set of skills and a no - nonsense attitude. In a Highschool DxD Dragonborn fanfic, he could be brought in to help train the dragonborn or fight alongside them against common enemies.
Vali himself is often a very popular character. His cool and powerful demeanor makes him a central figure in many fanfictions.