The fanfic might also explore how Issei's powers are affected by the transformation. In the original Highschool DxD, Issei has his own set of abilities as a male. But as a girl, he could either have new powers related to femininity in that universe or his existing powers could be modified. This could lead to new battles and challenges where he has to learn to use his powers in a different way.
One unique feature is the blend of two different and popular genres. The Highschool DxD's supernatural and often racy elements mix with the more cutesy and magical elements of the magical girl genre. For instance, you might see characters with devilish powers also using magical girl - like spells or transformations.
A possible plot could be Issei turning into a girl and having to hide it from his friends at first. This would create a lot of comical situations as he tries to maintain his normal life while dealing with the physical changes. For example, he might struggle with wearing female clothes or using the girls' bathroom without getting caught.
A third option could be 'Magical Girl in Highschool DxD World'. It may explore how a new character, a magical girl from another dimension perhaps, enters the Highschool DxD universe. This could cause a stir among the existing characters and lead to various comical, action - packed, or even romantic situations.
The Highschool DxD manga has indeed finished. It wrapped up after completing all the planned story arcs and providing a satisfying conclusion for the readers.
If Spawn were to interact with the main characters, he would bring a whole new level of seriousness. He has a dark past and a lot of baggage. When it comes to Issei's goal of becoming a harem king, Spawn might find it absurd but also interesting in a way. He could offer his own perspective on power and how to use it responsibly, which would be a contrast to the more carefree attitude of some of the Highschool DxD characters. And with the other characters like Akeno and Koneko, he'd be sizing them up, trying to figure out if they're threats or allies.