He would probably interact with Goku. Goku loves strong opponents, and Saitama's power would surely draw his attention.
Saitama could also interact with Beerus. Beerus is a powerful god of destruction. When he sees Saitama's strength, he might be both amused and a bit threatened. Their encounter would be quite comical yet also full of tension, as Saitama doesn't really care about Beerus' status and just wants to see how strong he is.
Saitama might interact with Vegeta as well. Vegeta is always striving to be the strongest, and Saitama's strange power would either make him want to fight or study Saitama. Also, he could interact with Whis. Whis is very knowledgeable, and seeing Saitama's power, he might be curious about where it comes from and how it works.
It could be really interesting. Saitama is extremely powerful in his own universe, and in a Dragon Ball Super fanfiction, he might be a wild card. Maybe he'd show up during a big tournament and shock all the Saiyans with his effortless strength. His simple, unassuming personality would contrast with the more flamboyant Dragon Ball characters.
Goku is, without a doubt, one of the most popular. His constant pursuit of strength and his ability to overcome seemingly impossible odds make him a favorite for fanfiction writers. They can create all kinds of new adventures for him, like facing off against cosmic threats even greater than what he's encountered in the shows.
It's hard to say. Goku in Dragon Ball Super has Ultra Instinct which gives him incredible speed and reflexes. But in GT, his Super Saiyan 4 form is also extremely powerful. However, if we consider the fact that Ultra Instinct seems to be a more refined and spiritual form of power, it might give Goku in Super an edge. But then again, the Super Saiyan 4 form has its own advantages like the combination of Saiyan and Great Ape power.
They could interact through training sessions. For example, Goku could teach some of the Naruto characters how to better control their energy, similar to ki control. In return, Naruto characters could show Goku and others some of their unique jutsu. In a harem setting, the female characters from both sides could be observing and learning as well, which would create interesting interactions and potential relationships.
Goku would probably be really excited to meet Luffy. He'd be curious about Luffy's rubber powers and would want to have a friendly sparring match right away. Luffy, on the other hand, would be in awe of Goku's strength and would see him as a great ally. He'd probably ask Goku to join his crew, but Goku would decline because he has his own friends and family in Dragon Ball world.
Goku would be really excited to meet all the different heroes in the DC universe. He'd probably want to spar with everyone right away. Flash would be intrigued by Goku's ability to move at high speeds using his Ki. They could have a friendly race or competition to see who's faster in different situations. At the same time, Piccolo could team up with Martian Manhunter. They both have the ability to sense things from a distance and could use their skills to detect threats to both universes.
I think Android 18 would be a fascinating character for Android 21 to interact with. Since they are both androids, they could share their experiences about being created and their place in the world. Android 18 might also be able to help Android 21 deal with her hunger for souls, as she has her own moral compass. Also, Trunks could be an interesting character. His time - traveling adventures and his determination to protect the future could intrigue Android 21. He could also introduce her to the concept of changing the future through one's actions. And last but not least, Mr. Satan. His over - the - top personality and his fame in the Dragon Ball Z world could provide some comical moments when interacting with Android 21.
Another likely pair could be Vegeta and Batman. Vegeta is proud and calculating, much like Batman. Batman would be intrigued by Vegeta's Saiyan pride and his combat skills. Vegeta, on the other hand, might respect Batman's strategic mind and his ability to fight without superpowers. Their first interaction could be a tense one, as Vegeta may not immediately trust Batman, but it would be interesting to see how they develop a relationship over time.
He could interact with Tsukune, the main human character in the Yokai Academy setting. They might share their experiences of being different in their respective worlds.
Vegeta is also highly popular. Writers often like to write about his pride as a Saiyan and how he overcomes his own ego to become a better fighter and a better person. His relationship with Bulma and his rivalry - turned - friendship with Goku are rich areas to explore in fanfiction.