A possible plot could be that Issei is transported to an alternate dimension where the concept of the Prince of Darkness is very different. He has to adapt to this new world while also trying to find a way back home. Along the way, he meets versions of his friends and enemies that are quite different from what he knows, and he has to use his wits and powers to survive.
One plot idea could be Issei facing a powerful new enemy that challenges his status as the Prince of Darkness. He might have to gather allies from different realms and use his unique powers in new ways.
A plot could involve the Prince of Darkness in a battle against other supernatural forces. He has to team up with unexpected allies, like a group of young witches who are also trying to protect their world from a greater evil. This would lead to a lot of character development as they learn to work together.
One idea could be Percy discovers he has a hidden connection to the powers of darkness that dates back to an ancient prophecy. Maybe he has to learn to control this new aspect of his powers while still being true to his friends at Camp Half - Blood.
A third plot idea is that there is a prophecy about the 'Prince of Darkness' in the Danny Phantom universe, and Danny is unknowingly fulfilling it. As he continues on his adventures, he starts to notice strange coincidences that seem to be leading him towards a dark destiny. He then has to decide whether to fight against the prophecy or try to understand it and use it for good.
One idea could be exploring the backstory of a minor character in 'into darkness'. For example, a crew member who has a hidden past related to the main antagonist. Another could be a time - travel plot where a character goes back in time to prevent the events that led to the 'darkness' in the story. Maybe also a story about a character who discovers a new power or ability that could change the course of the events in the 'into darkness' universe.
Another interesting concept could be that Izuku's 'prince of darkness' status is a hereditary thing that has been hidden from him all his life. When it starts to manifest, he has to deal with not only the physical changes but also the political intrigue within the world of dark powers, where different factions either want to use him or destroy him.
Since it's a fanfiction about Issei and 'the darkness', it might be a story that delves into the darker aspects of Issei's character. It could be set in a world where 'the darkness' is a powerful and menacing presence. Issei could be on a journey to either embrace or overcome this 'darkness'. There could be other characters involved who either help or hinder him in this exploration of the 'darkness' within and around him.
One idea could be their married life facing challenges from the supernatural world. Maybe a new powerful enemy emerges that targets their relationship. Another could be a story about their attempts to start a family within the context of the supernatural realm, with all the unique difficulties that would entail.
Well, there could be a story where Issei gets injured badly, and Ravel takes care of him. During this time, they start to see each other in a new light. It could also be about them participating in a tournament together, and their teamwork and growing bond make them a force to be reckoned with.
One plot idea could be a magical adventure where Issei and Yubelluna team up to save their world from a dark force. They could discover new powers along the way and their relationship could deepen as they face challenges together.
One plot idea could be Issei discovers a hidden relic related to King Arthur. This relic gives him some of Arthur's powers, leading to a power - up journey where he has to learn to control these new abilities while also facing enemies who want the relic for themselves. Another idea is that Issei somehow travels back in time to the era of King Arthur and has to interact with the legend himself, maybe even joining Arthur's knights for a while.