Their relationship could start as comrades in arms. As they face challenges together, they begin to respect each other's skills. Then, gradually, they might develop a friendship based on mutual trust. Eventually, it could turn into a romantic relationship as they discover more about each other's personalities and what they truly care about.
One interesting plot idea could be Issei and Le Fay teaming up to face a new, powerful enemy that neither of their usual allies can handle. They could combine Issei's power of the Boosted Gear with Le Fay's magic in unique ways. Another could be a story where they accidentally end up in an alternate dimension and have to find their way back while also dealing with the strange rules and creatures there.
It could be a new character introduced specifically for the fanfiction. Maybe a young witch or wizard who has been living in seclusion and discovers their heritage as the heir of Le Fay later on.
Yes, it could be considered in some ways. There are elements in the Harry Potter series that might show some inspiration from the concept of Lord Le Fay. For example, the magical worldbuilding and some of the magical elements might have been influenced by such related ideas. However, it's also a very unique and independent story with its own complex lore and characters.
There could be. A lot of fanfic writers come up with unique and creative relationships in their stories. Given the popularity of the Harry Potter universe and the allure of Morgana le Fay from Arthurian legends, it's not far - fetched that someone would write a fanfic exploring this relationship. However, it may take some time to find it as there are thousands of fanfics out there.
In a Naruto and High School DxD fanfiction, there could be a scenario where the protagonists of both stories meet in a neutral dimension. Issei from High School DxD might be intrigued by Naruto's ninja skills and determination. Naruto, in turn, could be curious about Issei's Sacred Gear. They could start training together, sharing their knowledge and skills, and ultimately have to face a common enemy that threatens both their worlds.
I'm not sure if there is such a specific fanfiction. There are countless Harry Potter fanfictions out there, but one with this rather dark and unethical - sounding plot might not be very common. It also might go against the general positive and moral themes of the Harry Potter universe.