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.
Natsu and Akeno could have some fun interactions. Akeno has her own brand of mischief, and Natsu might not be used to her teasing ways. Their first meeting could be full of misunderstandings, with Natsu thinking she's an enemy at first because of her devious appearance, but then realizing she's an ally and developing an interesting friendship.
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.
Naruto and Rias would be really interesting. Naruto's straightforward and determined personality combined with Rias' leadership and strategic mind could lead to some great interactions. They could learn from each other's fighting styles and ways of dealing with problems in their respective worlds.
Definitely Naruto himself. His positive attitude and never - give - up spirit would contrast greatly with the more morally ambiguous characters in Highschool DxD. He could also introduce the concept of chakra and ninja techniques, which would be completely new to the Highschool DxD world. His rasengan, for example, would be a very powerful and unique ability in that universe.
Itachi and Rias would be really interesting. Rias is used to being in control and having power, but Itachi's calm and collected nature along with his overwhelming power would put her in a different position. She would likely be intrigued by his Sharingan and his combat abilities. Itachi, on the other hand, would be dealing with a different kind of power structure and would have to navigate the social hierarchy of the devils in Highschool DxD.
Naruto himself would be a great choice. His sunny personality and strong will would contrast with the more complex and often morally ambiguous characters in Highschool DxD. He could bring a sense of innocence and pure determination to the world of DxD, and his unique jutsu would be a new and exciting addition to the battles there.
Ichigo and Akeno could also be an exciting pairing. Akeno has her own unique powers and a mysterious side. Ichigo's straightforwardness and strong moral code could contrast with Akeno's more complex nature. They could work together in battles, with Ichigo providing the brute force and Akeno using her strategic and powerful attacks.
One character that would make for an interesting pairing with Doflamingo in a crossover fanfiction is Zoro. Zoro's strong sense of justice and his skills as a swordsman would clash with Doflamingo's devious ways. They could have epic sword battles, and Zoro might be the one to cut through Doflamingo's strings, both literally and metaphorically. Also, Nami could be paired with Doflamingo. Nami's intelligence and ability to manipulate situations for her own gain, although in a more positive way compared to Doflamingo, could lead to some really interesting interactions. She could try to outwit him in a game of wits.
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.
Ichigo and Rias would be a very interesting pairing. Ichigo is a powerful and determined fighter, and Rias is a strong - willed Devil. Their personalities could clash at first, but then they might find common ground in their leadership roles and their desire to protect their respective groups.