Another theme is the clash of cultures. Naruto comes from a world of ninja clans and hidden villages, while Ulquiorra is from a more spiritual and otherworldly setting. Fanfictions use this contrast to create interesting storylines, like how the two characters might misunderstand each other's values at first but then gradually learn to respect them.
Ulquiorra is a very interesting character to analyze in Highschool DxD fanfiction. His stoic and often emotionless nature from his original Bleach series can create some really unique interactions in the Highschool DxD universe. For example, his power level and combat skills would likely make him a formidable force in the DxD world. His unique Hollow abilities could be adapted in various ways, like his Cero could be seen as a more powerful version of a devil's attack.
Yes, there are likely fanfictions about Ichigo fighting Ulquiorra that are popular. 'Bleach' has a huge fan base, and the fight between Ichigo and Ulquiorra was a very significant moment in the series. Fans often create fanfictions based on such iconic battles, exploring different scenarios, character emotions, and outcomes that might not have been shown in the original anime or manga.
In a fictional context, it can be ethical as long as it adheres to certain moral boundaries within the fandom. For example, if the story is more about the psychological battle between Ulquiorra and the female prisoner rather than just gratuitous torture scenes. But if it goes too far and uses the torture in a way that is disrespectful to the idea of human dignity, especially of female characters, it crosses the line.
There's also 'Naruto and Ulquiorra: Unlikely Allies'. It focuses on how Naruto's positive and determined nature starts to influence Ulquiorra, who is initially quite cold and indifferent. It's a story full of character development and exciting battles as they face common enemies together.
It would be really wild. Maybe Harry Potter would have some of Ulquiorra's powers like his cero blast, but in the form of a wizard spell. Or perhaps he would have Ulquiorra's cold and emotionless demeanor in the wizarding world, which would be a huge contrast to his normal self. He could be dressed in a way that combines the Hogwarts robes with Ulquiorra's Arrancar outfit.
Ulquiorra could also be a part of a secret organization in the Highschool DxD fanfiction. This organization is interested in his powers and wants to use them for their own ends. Ulquiorra has to decide whether to go along with their plans or break free and find his own path in this new world filled with strange beings and even stranger politics.
The main appeal is likely the expansion of the 'Bleach' universe. The fight between Ichigo and Ulquiorra was a key moment. In this fanfiction concept, with everyone watching, it can show the significance of the fight on a larger scale. It can also be used to develop minor characters who are just spectators in this scenario. Their reactions, thoughts, and any actions they might take because of the fight can make for an interesting read and add more complexity to the 'Bleach' world.
I'm not entirely sure it's a very common or legitimate concept. Harry Potter and Ulquiorra are from two very different fictional universes. Harry Potter is a wizarding world with its own set of rules, magic, and characters, while Ulquiorra is from Bleach, a completely different anime universe. Fanfiction usually combines elements within the same or related universes more often. So it might be a very niche or even an odd and not - so - recognized concept in the broader fanfiction community.
You should not be looking for 'lemon' fanfics as they often violate the terms of service of most platforms. Instead, look for regular 'orihime x ulquiorra' fanfics. You can start by using search engines and typing in the relevant keywords along with the name of the series. This might lead you to some fan - made blogs or websites that host such fanfics. But always be careful about the legality and propriety of the content.