An exiled God. He was banished from his own pantheon for some misdeed. In the Highschool DxD world, he is trying to find a way to redeem himself. He has unique powers that are different from the other characters, and his story could be one of redemption and self - discovery as he interacts with the devils, angels, and fallen angels in the story.
How about a character who is a descendant of an ancient dragon - slaying clan? This character might initially be hostile towards the Vanishing Dragon and its wielder because of their clan's history. But as the story progresses, they could come to understand the difference between the Vanishing Dragon and the ones their clan used to slay, and end up becoming an important ally. They could bring in new fighting techniques and knowledge about dragons.
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.
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 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.
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.
A young and talented artist could be a new character. The empress could commission the artist to create a portrait or some art for the palace, and in the process, a unique friendship or a complex relationship could develop. The artist could also be a source of inspiration for the empress in different ways.
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.
One potential new protagonist could be Issei's other friends or comrades. Since Rias is being bashed in this type of fanfiction, they might get more screen time and development. For example, Kiba could step up and show his leadership skills and unique abilities more.