There's a fanfic where the Egyptian gods interact with the devils in Highschool DxD. In this story, the gods like Ra are portrayed as very powerful entities. They come to the world of Highschool DxD to check on the balance of power. It has a lot of action scenes and interesting character interactions.
Clash of the Titans was a movie series about the battle between the son of Zeus and the other gods. The series included 2010's Clash of the Titans and the 2012's follow-up, Rage of the Titans. The story was about the adventures of Perseus, son of Zeus, and the other gods to save the world. The series was directed by different directors and had a strong cast, including Sam Worthington, Liam Neeson, and Ralph Fiennes. The movie showed the fierce battle and rescue operation between gods and humans.
It's unique because it allows for a different take on the power dynamics. Egyptian gods often have different types of powers compared to the regular Highschool DxD characters. For example, the power of resurrection or control over the elements in a more ancient and primal way. This can lead to very interesting battles and strategic situations in the fanfiction. In one fanfic I read, the character with the power of Osiris was able to bring back characters from the dead, which completely changed the course of the story and the relationships between different factions.
There's a fanfiction where Issei's son has a unique power that sets him apart. In this story, he has to learn to control it while also handling the typical Highschool DxD drama like rivalries and romances. The author does a great job of showing how he struggles with being in his father's shadow at times, but also rises above it in his own way.
I haven't read this particular fanfiction yet, but generally, a fanfiction about the son of Sirzechs in the Highschool DxD universe could be really interesting. It might explore new powers, relationships, and storylines that the original series didn't cover. For example, how the OC would interact with the existing characters like Issei and Rias, and how his heritage as the son of Sirzechs would shape his destiny in the Devil society.
You can try websites like Fanfiction.net or Archive of Our Own (AO3). These are popular platforms that host a large number of fanfictions, including those related to Highschool DxD with god - related themes.
There are quite a few. For example, some fanfictions could depict gods from different mythologies coming together in the Highschool DxD setting. They might be involved in battles, or they could be there to offer guidance to the characters. You really have to dig around on the fanfiction platforms to find the really good ones though. A lot of them play with the idea of the gods' powers and how they compare to the abilities of the regular Highschool DxD characters.
The OC, as the Son of Sparda, could fit in by being a new and powerful entity. He might be initially seen as an outsider, but his powers would quickly draw attention. He could become involved in the power struggles between the different factions in Highschool DxD. For example, the devils might try to recruit him, while the angels might be wary of his presence. His unique abilities would also make him a valuable ally or a fearsome opponent. And in the high school setting, he could be a bit of an enigma to the other students, both human and supernatural.
The idea of Naruto being the son of Greek gods is really creative. His journey would be different. For example, he could have to go on quests like the heroes in Greek mythology. His friends in the ninja world might be confused by his new powers and the strange things that start to happen around him. It would be a story full of adventure, as he tries to balance his two heritages and save both the ninja world and maybe even parts of the Greek mythological world.
Romance is also a big theme. Just like in the original Highschool DxD, his son usually has his own love interests. These relationships can be complicated by his status as Issei's son. Sometimes, potential partners are attracted to him because of his family name, and he has to figure out if their feelings are genuine.