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.
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.
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.
One new character could be a Diabolos Dragon knight. This character has sworn to protect the Diabolos Dragons at all costs and initially sees Issei as a threat. As the story progresses, they might become friends. A female Diabolos Dragon with the ability to shape - shift into human form could also be introduced. She could be a love interest for Issei or a rival for his affections. And then there's the Diabolos Dragon hatchling that Issei finds and decides to raise, which brings a whole new dynamic to the story.
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.
Well, a plot could involve the Vanishing Dragon's power being misused by an evil force. The protagonist then has to go on a journey to reclaim and purify that power. This could involve teaming up with some of the characters from the original series, like Issei and his friends. They would have to overcome various challenges and battles, all while learning more about the true nature of the Vanishing Dragon.
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.
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.