I think Nami could be fascinating. In a DxD setting, she could use her wits and charm to manipulate some of the Devils and Angels, just like she does with pirates in One Piece. Also, her knowledge of navigation and treasure hunting could give her a unique role in the DxD universe, perhaps leading her to discover some hidden treasures related to the different factions.
Luffy would be really interesting. His carefree and adventurous nature would make his participation in the Grand Magic Games full of surprises. He'd probably approach the magic - based challenges in his own unique, unorthodox way, like trying to stretch his body to reach magic items instead of using proper spells.
Luffy and Issei would be an interesting pair. Luffy's carefree and fearless nature combined with Issei's determination and his unique Sacred Gear abilities could lead to some really fun and action - packed scenes. They could learn from each other, like Luffy teaching Issei about his unwavering belief in his dreams and Issei showing Luffy some of the more strategic ways to use powers in the Highschool DxD world.
I think Naruto himself would be the most interesting. His positive attitude and never - give - up spirit would be really unique in the Highschool DxD world which has a lot of complex power struggles and some rather morally grey areas. He could bring a fresh perspective and also his powerful ninja techniques would make him a force to be reckoned with in battles against devils and angels.
I think Naruto would be the most interesting. His energetic and determined personality would stand out in the Highschool DxD world. He could bring his ninja way of life and values, which are quite different from the devilish and angelic cultures in DxD. His ability to make friends easily might also lead to some really unique interactions with the DxD characters.
Rias from Highschool DxD would be an excellent choice too. Her leadership skills and her role as a devil could be explored in relation to the Naruto characters. She could be intrigued by Naruto's power and try to recruit him into her peerage, or she could learn from Naruto's way of leading his friends and comrades in battle.
I think Naruto himself would be really interesting. His never - give - up attitude and his ability to make friends easily could have a big impact in the Highschool DxD world. He could also learn a lot from the different power structures there. Plus, seeing him interact with the devils and angels would be really cool. He might try to convince them to use their powers for more positive things, like he did with some of the villains in his own world.
Rias Gremory would also be an interesting character to focus on. If she were to interact with Ichigo, her leadership skills and her own goals within the Highschool DxD world could be influenced. Maybe she tries to recruit Ichigo for her peerage, which would lead to all sorts of conflicts and negotiations.
Issei could also be a very interesting character to focus on in this fanfic. He is the main character in Highschool DxD. If Naruto and Kuroka enter his world, Issei's reaction to them would be fascinating. He could learn a lot from Naruto's discipline and hard - working nature. And his relationship with Kuroka could also change in new and unexpected ways, adding depth and drama to the fanfic.
Definitely Batman and Luffy. Luffy's carefree and positive attitude would contrast with Batman's brooding nature. But they both have a strong sense of justice. Luffy punches bad guys, and Batman uses his gadgets and martial arts. They could learn a lot from each other. Another interesting pair could be Robin and Alfred. Alfred's wisdom and knowledge could help Robin understand more about the history and culture in Batman's world, while Robin could share her knowledge of the Poneglyphs with Alfred.
Frisk would also be a great character to focus on. If they were to gain some of Saitama's power, it would completely change their journey in the Underground. They could use their new strength to protect the monsters instead of just trying to find a way out. This would lead to new relationships and storylines. They might be able to stand up to Flowey in a different way, and even change the way the entire Underground views humans.