In Highschool DxD multiverse fanfiction, character development can take on new dimensions. New relationships can be formed between characters from different universes. This can lead to some really creative storylines. For instance, a character from a more peaceful universe might have to team up with a battle - hardened character from another universe, and they learn from each other's experiences.
Another popular theme is alternate relationships. The multiverse allows for different pairings between characters that might not be possible in the original story. It gives a fresh perspective on the characters' personalities and how they interact with others.
You can try fanfiction websites like FanFiction.net or Archive of Our Own (AO3). They usually have a wide variety of fanfictions, including those related to Highschool DxD and the multiverse concept.
Well, in some fanfictions, they could travel to a multiverse where the rules of the Highschool DxD world are completely different. For example, the devils, angels, and fallen angels might have different hierarchies or abilities. This can lead to really exciting plot twists as the characters have to adapt to these new environments and find a way to get back to their own universe.
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.