Well, it might be someone like Vali. He has his own sense of justice and is extremely powerful. In fanfiction, the author could easily shape him into a heroic heir. He has a complex personality and his interactions with Azazel could lead to a great storyline with him as the heroic figure.
The heroic heir should have a strong sense of justice. Just like the main characters in the original DxD series. They need to be able to distinguish right from wrong and fight for what is good. Additionally, they would likely possess some kind of special power or ability. This could be something they are born with or something they inherit from Azazel or other sources. This power would help them in their battles against evil. For example, it could be a unique form of magic or a powerful sacred gear. Moreover, they would need to be a good leader. Since they are an heir, they may have to lead others in battles or in the pursuit of a greater good.
Naruto is often a very popular character in these crossovers. His strong - willed nature and powerful ninja abilities make him stand out. He can bring a new level of action and adventure to the Highschool DxD setting.
Friendship and loyalty can also be themes. He may form new friendships or test the loyalty of existing ones. For instance, he could be in a situation where he has to decide between his loyalty to his fallen angel comrades and a new alliance with other species.
Yes, there are many out there. You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Naruto Azazel High School DxD' and you'll find a bunch. Some might be crossovers that combine the elements of Naruto and High School DxD in interesting ways with the character Azazel.
One interesting crossover could be the combination of Naruto's ninja skills with the supernatural elements in Highschool DxD. For example, Naruto's shadow clone jutsu might be used in a unique way during a battle in the Highschool DxD universe. Maybe the clones could be used to distract devils or fallen angels while the real Naruto uses his Rasengan.
One common theme is power. Since both Naruto and High School DxD have complex power systems, fanfictions often explore how the characters' powers interact. Another is friendship, as in how Naruto's idea of friendship might influence the relationships in High School DxD. And of course, there's the theme of adventure, with Naruto getting involved in the wild adventures that usually happen in High School DxD.
It could be a powerful mutant who wants to use Nightcrawler's teleportation abilities for his own evil purposes. He might see Nightcrawler as a key to achieving some sort of world - domination plan.
One interesting Azazel fanfiction could be one where he mentors the main character in a more in - depth way, exploring his knowledge and skills. For example, a story where he takes the protagonist on a journey through different dimensions to teach about the various supernatural elements in the Highschool DxD universe.
In such a fanfiction, Naruto being the son of Azazel would bring a whole new dynamic. His chakra could be seen as a unique form of power in the Highschool DxD universe. He could potentially use it to enhance his physical abilities even more than the normal characters in that world. And his rasengan? It could be a really powerful attack against the various creatures in the story. He might also have some internal conflict about his heritage, being part - ninja and part - fallen angel.
He would probably have Naruto's regular ninja powers like the rasengan and shadow clones. But combined with his Azazel heritage, he might also have some unique fallen - angel - like abilities. Maybe he could fly or have some sort of dark energy - based attack.