It could be a new student who discovers they are born of fire. Maybe they were adopted and had no idea about their true nature until they start experiencing strange powers during their time at the Highschool DxD. This character would then be thrust into the world of devils, angels, and other supernatural beings.
One interesting plot idea could be a new character born from the fire element who has unique powers that disrupt the balance in the Highschool DxD world. For example, they might be able to control a type of fire that is immune to the usual magic defenses. Another idea could be a story where the main character discovers a hidden history related to the 'born of fire' concept in the Highschool DxD universe, like an ancient prophecy or a long - lost clan.
In fanfiction, anything can happen. The author might have created a crossover concept where Naruto's abilities are combined with elements from Highschool DxD. Maybe it's a result of some magical or scientific experiment in the fictional world of the fanfic. It could also be a unique power that Naruto acquires during his adventures in the Highschool DxD universe.
It could be used for combat. The black fire might have properties like burning through magical barriers or causing extra damage to supernatural beings in the Highschool DxD world. For example, it could be really effective against devils or fallen angels.
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.