Well, he could be a powerful antagonist. His skills and power from the Dragon Ball universe would make him a formidable foe in the High School DxD world. He might be interested in the power of the devils and angels there and try to take it for himself.
Goku Black could potentially enter the High School DxD world through a dimensional rift or some sort of magical accident. Once there, his Saiyan pride and his dark motives (from his own story) could lead to him either trying to take over the various factions in the DxD world or simply looking for a worthy opponent. He might interact with the main characters like Issei, perhaps seeing him as a target to prove his superiority or a potential ally in his own schemes.
Goku Black might become a sort of anti - hero. He has his own sense of justice (albeit a twisted one in his original story). In the High School DxD world, he could oppose some of the more corrupt elements of the different factions while also causing trouble for the main characters as they try to figure out his motives. For instance, he could see the way some devils treat humans in the DxD world and decide to take matters into his own hands, but not in a way that the main characters would necessarily approve of at first.
His ki - based attacks would be a new and powerful addition. In High School DxD, most of the powers are related to magic, sacred gears, and demonic or angelic powers. Goku Black's ki blasts would be a completely different form of energy that could catch everyone off - guard.
Goku's powers are on a completely different level compared to most characters in Highschool DxD. His Saiyan abilities like Super Saiyan transformations give him incredible strength, speed, and energy projection. In contrast, the characters in Highschool DxD mainly rely on magic and their devil or angelic powers which, while powerful, are not on the same scale as Goku's in terms of raw power. For example, Goku could easily defeat most low - level devils with a single punch in his base form.
Issei would be very likely. Since Issei is the main character in DxD, his curiosity and his own pursuit of power would make him interested in Goku. They could train together and learn from each other's abilities. Also, Rias would interact with Goku. As a powerful devil, she would be intrigued by Goku's power and his different way of approaching battles.
Yes, there are some great ones. You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Goku Highschool DxD' and you'll find a variety of stories with different plotlines and interpretations.
Well, there are several common themes. First, there's the clash of cultures. Goku comes from a very different background than the characters in Highschool DxD, so how they interact and understand each other is a big part of many stories. Then there's the theme of self - discovery. Goku might discover new aspects of his powers or his personality while in this new world. Additionally, the idea of protecting the new world he's in is also quite common. Since he has a heroic nature, he would likely want to keep the Highschool DxD world safe from any threats, whether they're new to him or familiar from his own universe.
Goku's powers in a High School DxD fanfiction would be a game - changer. His ki attacks could be on a level that could potentially destroy entire areas in the DxD universe. His martial arts skills are also top - notch. In a fight, he would use his speed and agility to dodge attacks and then counter with his powerful punches and kicks. His transformation abilities, like Super Saiyan, would be a shock to the characters in DxD. They might not have any means to counter such a huge boost in power.
Zamasu is often paired with Goku Black. Since they are closely related in the original story, fanfiction writers like to explore their relationship further. It could be about their shared goals or how they interact on a more personal level.