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.
The Skull Kid from Majora's Mask could be the main character. Imagine him being transported to the Highschool DxD world. He could cause all sorts of mischief with his magic. He might not understand the complex social structures in the high school, like the relationships between the devils, angels, and fallen angels. His interactions with the students and his attempts to find a way back to his own world could be really interesting to explore.
One possible plot could be that the protagonist from Highschool DxD somehow gets transported to the world of Majora's Mask. He might have to face the strange and menacing entities there while also trying to find a way back home. Along the way, he could use his unique abilities from Highschool DxD, like his demonic powers, to combat the threats in the new world. Maybe he forms an alliance with some of the characters from Majora's Mask, such as the Skull Kid, who could help him understand the strange rules of this new realm.
Issei would probably be one of the main characters as he is a central figure in DxD. He might be the one to first encounter elements from Majora's Mask. Link could also be a main character, bringing his skills and knowledge of the Zelda universe. Rias could be involved too, as she is an important character in DxD and her leadership skills and powers would be relevant in a crossover. And maybe Skull Kid, from Majora's Mask, could play a significant role, causing chaos with the power of Majora's Mask in the DxD world.
One interesting concept could be the combination of the unique and often dark atmosphere of Majora's Mask with the high - energy, supernatural world of DxD. For example, the masks in Majora's Mask might have different effects when used in the DxD universe, like the Deku Mask giving a character plant - based powers in the context of the devil - filled world of DxD. Another concept could be the interaction between the characters from both. The protagonists of each might form unexpected alliances or rivalries. Maybe Issei from DxD could be intrigued by the power of the Fierce Deity Mask and try to find a way to use it for his own battles against the devils and fallen angels.
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.
Definitely Sora from Kingdom Hearts. His Keyblade skills and experience in different worlds would make him a great addition. He could help Link in the fight against the forces of Majora. Another good character would be Mario. His jumping abilities and general heroics could be put to good use in Termina. And then there's Kirby. Kirby could absorb the powers of the enemies in Majora's Mask and use them in unique ways to help out.
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.
Sakura could be an interesting crossover character. In Highschool DxD, she could show off her medical skills which are quite advanced in Naruto. She could also develop her combat skills further by learning from the DxD characters. Her strong - willed personality would make her a force to be reckoned with, and she could get involved in the peerage system, perhaps even leading her own peerage.