A key element is the power contrast. Superman is extremely powerful compared to many characters in High School DxD. So how his powers are portrayed and how they interact with the existing power systems in DxD is important. For example, his super strength and flight can change the way battles play out.
Some characters might be critical. If they have a strong sense of the 'canon' Superman, they could nitpick at the details in the fanfic that don't match up. For instance, if the fanfic changes Superman's origin story too much, they might frown upon it.
If Superman were in Highschool DxD fanfiction, the power dynamics would be drastically altered. Superman's super strength, speed, and heat vision would make him an overpowered character. He could easily defeat most of the foes in the Highschool DxD universe. This would likely change the plot as many of the story arcs in Highschool DxD revolve around the characters gradually growing in power and facing challenges that match their strength. It might also lead to different character interactions. For example, Superman's moral code might influence the more morally ambiguous characters in the series.
The characters in DxD would be quite shocked at first. Ichigo's appearance and his powers are so different from what they are used to. For example, Rias Gremory would probably be intrigued by Ichigo. She might try to form an alliance with him, seeing his potential as a powerful ally. Issei, being the main character in DxD, would likely be both jealous and in awe of Ichigo's abilities, and might challenge him at first but then learn from him over time.
Another great fanfiction could be 'When Superman Joins High School DxD'. Here, Superman decides to enroll in the school in the DxD world. His actions and decisions, guided by his strong moral compass, create a lot of interesting situations. It's a story full of adventure, as he has to deal with the various challenges and temptations that the DxD world presents while staying true to himself.
One idea could be that Superman, with his powers, becomes a sort of protector in the High School DxD world. He could use his super strength to stop any over - the - top fights that break out between the different factions. And his heat vision might come in handy when dealing with some of the more dangerous magical threats that lurk in the shadows of the school.
Well, if Superman were in High School DxD fanfiction, the power dynamics would be completely shifted. Superman's super strength, flight, and heat vision would make him a dominant force. He might be seen as a rival to the devils and angels in the DxD universe. His moral code would also likely influence the more hedonistic characters in the series.
In the school in Highschool DxD fanfiction, Superman could be a protector. He would use his powers to keep the students safe from any threats, whether they are from the supernatural world or just normal human dangers like a fire or a bully. He might also be a role model for the students, showing them what it means to have a strong moral compass.
Issei would probably be the first to want to fight Thanos. He's always eager to prove himself and protect his friends. He might initially underestimate Thanos due to his own growing power and confidence, but soon realize how powerful Thanos really is. Rias would be more cautious. She would likely try to gather information about Thanos first and then formulate a plan to either fight or negotiate with him.
Issei, being the hot - blooded character he is, would probably be excited to meet someone as powerful as Naruto. He might challenge Naruto to a friendly spar at first, hoping to learn from him and test his own strength. And as he gets to know Naruto better, he could start to see him as a brother - in - arms, especially if they face common enemies together.