A female cultivator could be the main character. She might be on a quest for revenge in her own world but gets sucked into the DxD world instead. In the DxD world, she has to start from scratch in a sense. She uses her wits and cultivation skills to survive and eventually rise to power. Her gender could also play an interesting role as she might face different challenges compared to a male character in a world that also has its own ideas about gender roles and power dynamics.
One possible plot could be a cultivator arriving in the DxD world and being mistaken for a powerful devil. They might have to navigate the complex politics of the three factions while also trying to find a way back home. Along the way, they could introduce some of their cultivation techniques to the local characters, which would cause a stir as they are so different from the magic and power systems already in place.
Itachi could be a great main character. His complex personality and powerful abilities would make for an interesting story in the DxD universe. He could be a sort of enigma to the DxD characters, and his Sharingan would give him an edge in battles against the various beings there.
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.
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.
It could be a powerful devil who wants to take over the entire world now that there is no God to oppose them.
It could be Issei. Since he's always eager to become stronger and protect his friends, his encounter with the Annihilation Maker could lead to some really interesting storylines. He might be drawn to the Annihilation Maker's power and seek to learn from them to gain an edge in battles against the various threats in the High School DxD world.
A human with a special power that makes them a killer could also be a good choice. They could be an outcast in the human world because of their power, and then get drawn into the High School DxD world. Their journey from being a feared killer to finding a place among the supernatural beings would be an exciting story to tell.