Definitely Issei. He's always looking to get stronger and Goku is like a walking encyclopedia of power - up techniques. Also, Azazel might interact with Goku. Azazel is a very intelligent character in DxD and would be interested in studying Goku's Saiyan biology and powers. And Vali, being another powerful fighter in DxD, would surely cross paths with Goku, probably to test his own strength against Goku's.
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.
Well, first of all, Issei. His nature as a perverted yet power - hungry character would make him want to be around Goku. He'd want to see if he could learn some of Goku's super moves. Then, there's Sirzechs Lucifer. As one of the most powerful devils, he would be interested in Goku's power level and might approach Goku to either form an alliance or just to understand his motives. And don't forget about Asia. Her kind - hearted nature would make her want to befriend Goku and perhaps heal his wounds if he gets injured during his adventures in the DxD world.
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.
Probably Goku and Twilight Sparkle. Twilight is the smart and curious one. She would be the first to approach Goku out of scientific interest, wanting to understand where he came from and what his powers are all about.
Well, Vegeta would be a likely character. He might be initially suspicious of the female Goku. Maybe he thinks she's a threat or just can't believe there's another Goku - like being. He could be there to add some comical moments as he tries to understand the situation.
Nora could also be a likely candidate. She's very energetic and not one to be intimidated easily. When she sees Goku's amazing power, she would probably be excited and want to team up with him right away. She would be constantly asking Goku about his world and his powers, and they would bond over their love for combat and having a good time.
Obviously, Goku and 17 are the main characters. Goku, the Saiyan hero with his incredible power and love for fighting. 17, the powerful android with his own set of skills and a somewhat mysterious past.
Vegeta from the Dragon Ball series is often popular. Fans like to write about his reaction to Naruto's world and how he might interact with the characters there. Also, Sasuke from Naruto is frequently included. He could be a rival or an ally in the fanfiction, given his complex character. Another one could be Piccolo. His calm and strategic mind would be interesting to incorporate in stories involving Naruto's more chaotic ninja world.
Vegeta could be a likely character. In some fanfictions, his pride and envy of Goku's strength might lead him to betray Goku. He might think that by getting rid of Goku, he can become the strongest in the universe. Another one could be Frieza. Even if he pretends to be an ally in some alternate storylines, he could betray Goku at a crucial moment to regain his power and dominance. Also, a new and powerful character introduced in the fanfiction, who is envious of Goku's popularity and power, could easily be a betrayer.
Goku would probably be very friendly with Luffy. They both love to fight strong opponents and have a sense of adventure. Goku might be a bit confused by some of the wacky things in the One Piece world like the Devil Fruits at first, but he'd quickly adapt. He'd also likely be impressed by the determination of the other characters like Zoro and Sanji.
One possible plot could be that Goku is somehow transported to the DxD universe. He might initially be confused by the different rules and magic there. Then, he could start training with the devils and angels, sharing his own fighting techniques. This could lead to some epic battles where he combines his Saiyan powers with the unique abilities in the DxD world.
Sasuke could also oppose Black Goku. With his Sharingan and Rinnegan, Sasuke has a wide range of powerful abilities. He is also very intelligent and strategic. Sasuke would likely see Black Goku as a threat to the balance of power in the Naruto universe and would step in to fight him, especially if it involved protecting his friends or the ninja world in general.