Rias could also be the main character in a sense. If we consider a story from a different perspective where she is the one getting married and the focus is on her partner dealing with her father - in - law. Since she is a powerful devil herself, her family dynamics and the relationship with her father - in - law would be full of political and power - related implications in the Highschool Dxd world.
Maybe a new character could be the main character. A human who accidentally stumbles into the Highschool Dxd world and falls in love with one of the female characters there. Then he has to face the challenges of dealing with her father - in - law, who could be a powerful being like a devil king or a high - level angel.
Well, it could be a story where the father - in - law has some special mission or task for the main character in the fanfiction. Since Highschool Dxd has a lot of supernatural elements, perhaps the father - in - law wants the character to retrieve a powerful artifact or protect a certain person. And this forms the basis of their relationship development throughout the fanfiction.
Issei could be an interesting main character in a 'Highschool DxD Akatsuki' fanfiction. He already has his own journey in the Highschool DxD world, and if he were to interact with the Akatsuki, it would add a whole new dimension to his story. He could learn new techniques from them or even form an alliance against a common enemy.
The devils who oppose Grayfia's family could be the main antagonists. They might see Grayfia's brother - in - law as a weak link or a potential threat to their power. So they would try to get rid of him or manipulate him for their own gains.
A new character could be introduced as the main character. For instance, a transfer student who has this hypnotic power. He could be mysterious at first, and as he gets involved with the harem, he gradually reveals his true self and the reasons behind his power.
It could be one of the well - known kamen riders like Kamen Rider Decade. Since he has the ability to travel through different worlds, it would be plausible for him to end up in the world of Highschool DxD. He could use his powers to adapt to the new environment and deal with the various challenges there.
Issei would be a popular choice. Since he is the main character in the original series, having him time travel would create many interesting storylines. He could use his knowledge of future events and his growth as a character to change things in the past.
It could be an original character created just for the fanfiction. Maybe a nerdy gamer who has always dreamed of being in a world like Highschool DxD and suddenly finds himself there with incredible powers.
Issei could be the main character. He is already used to dealing with supernatural situations in Highschool DxD. So when faced with the strange world of Majora's Mask, he would be in a position to adapt and try to find solutions. His perverted nature might also add some comical elements as he reacts to the new and different characters in the Majora's Mask world.
Maybe a rogue ninja from Gaara's world who somehow follows him into the DxD highschool. This ninja could have a vendetta against Gaara and use the chaos of the DxD world to try and take him down. They could also team up with some of the devils who dislike Gaara's presence to create more trouble for him.
Issei could still be the main protagonist. His determination and ability to grow stronger could play a big role in any story involving Trihexa. He might be the one to face Trihexa head - on or try to befriend it.