One important aspect that could make this fanfiction great is the world - building. Since it's set in the Highschool DxD universe, the author should stay true to the lore while also adding new elements related to the OC. For instance, how does the fact that he's Sirzechs' son affect the power dynamics in the Underworld? Will he be involved in political intrigue? And also, his relationships with other species like the Angels and Fallen Angels could be a really exciting part of the story.
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.
Well, if it's a good fanfiction, the author would probably develop the character of the OC son of Sirzechs in a unique way. They could give him some of Sirzechs' powerful abilities, like his unique magic or leadership skills. Maybe the story could involve him facing challenges that are different from what other characters in Highschool DxD have faced, perhaps something related to his special status within the Devil world and how he has to prove himself despite his noble lineage.
If you want the clearest chapter reference, go to Chapter 195. Early on, Jiang Yan was fully equipped and followed at the end of the team.; later, the chapter leans on He was wearing a sports suit borrowed from He Yun, his face smeared with....
A practical answer is Chapter 206, because it turns the idea into a concrete scene. The chapter opens with Five minutes later, Xingxing led Ah Fu out of the room by the hand., adds pressure through Miss Su, you're even better than us parents!, and ends on He was the oldest of several siblings and.
The best chapter pointer here is Chapter 159, "The Other Flying Guest is Actually Him?!". It opens with Seeing the red light of the camera turn on, Su Yinwan looked at the... and closes on Shang Shaobai, who was browsing through files, received a call from his....
The best chapter pointer here is Chapter 210, "Are All of Jiang Li's Fans Hidden Big Shots?". It opens with This was Zhao Lang's first public appearance. and closes on Clearly, she used improper methods to get it..
Chapter 167 is the strongest chapter-level answer because it starts from By the experimental field, Qi Tianyu was quite helpless as he watched Wen... and follows through on Even when getting into the car, Qi Tianyu was still savoring the moment in....
A practical answer is Chapter 185, because it turns the idea into a concrete scene. The chapter opens with He was so anxious that he immediately picked up the megaphone., adds pressure through Shang Shaojing had not anticipated that Jiaojiao, who had slacked through..., and ends on The next, she'd.
A practical answer is Chapter 190, because it turns the idea into a concrete scene. The chapter opens with Several people who didn't understand fell silent., adds pressure through And it became even more repugnant to him., and ends on Because he saw the pack of cigarettes pinched between Jiang.
A practical answer is Chapter 193, because it turns the idea into a concrete scene. The chapter opens with Jiang Shixu's eyes lit up when he heard there was a gift., adds pressure through Does she really think the audience is blind?], and ends on Jiang Cheng was a prime example..
A practical answer is Chapter 168, because it turns the idea into a concrete scene. The chapter opens with Su Yinwan spoke those words with a pitiable expression, then lowered her..., adds pressure through But this time, for lunch, Jiang Li cannot help, the rest of you must..., and ends on The Gu.
Look to Chapter 210 for the clearest answer. It begins with This was Zhao Lang's first public appearance., keeps building through He was dressed in a set of black and white local attire, with a simple..., and lands on Clearly, she used improper methods to get it..