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.
Rias could also be a great main character for such a fanfiction. She has a strong personality and leadership skills. If she were to travel back in time, she could change the power dynamics within the devil society much earlier, perhaps preventing some of the rivalries or power struggles that occurred in the original timeline.
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.
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.
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.
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.
It could be a corrupted member of the mafia who has made a deal with a rogue group of fallen angels. This person betrays his own mafia family and works with the fallen angels to create chaos in the Highschool DxD universe. He is very sly and manipulative, always staying one step ahead of the heroes. For instance, he uses his knowledge of the mafia's operations and the power of the fallen angels to set traps for the protagonists.
Issei could be the main protagonist. Given his determination and his ability to grow stronger, he could be in the forefront of fighting against the zombies. He might use his Boosted Gear to not only battle the undead but also to find a cure if there is some sort of infection involved.
The devils who oppose Gilgamesh's presence in the Highschool DxD world could be main antagonists. They might see him as a threat to their power and influence in the school.
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.