Maybe they just saw some very superficial similarities. For example, both stories have elements of a journey. In Dante's 'The Divine Comedy', he goes on a journey through the afterlife, and in Harry Potter, Harry and his friends go on various adventures through Hogwarts and other magical places. But that's about as far as it goes really.
Maybe they were just being really creative in a very strange way. They might have thought about the contrast between the relatively 'tame' magic world of Harry Potter and the chaotic and violent nature of Carnage and thought it would be an interesting, if completely off - the - wall, combination.
Perhaps they are misinformed. They might not fully understand the nature of 'Harry Potter' as an original work. Maybe they saw some superficial similarities between 'Harry Potter' and elements in 'leviathan' fanfiction, but failed to recognize the vast differences in the overall concepts, themes, and story structures.
Perhaps it was a joke at first. Somebody who was really into both 'Harry Potter' and 'Rick and Morty' might have made the comment in a humorous way, not really meaning it literally. But then it got spread around as a strange sort of idea. Another possibility is that someone was trying to be edgy or different and just threw out this completely unorthodox concept without much thought behind it.
It could be that the person has a very different view of how fictional works can be related. They might see some very minor similarities in the character development arcs or the idea of a hero's journey in both Harry Potter and Naruto and then take it to an extreme by suggesting such a thing. However, when you look closely at the details of each series, it becomes clear that it's a baseless idea. For example, Harry Potter's journey at Hogwarts is about learning magic and fighting dark wizards, while Naruto's is about becoming a ninja and uniting the ninja villages. They are very different in nature.
It could be due to a complete misunderstanding. Maybe they were thinking about ancient or old - fashioned elements and randomly associated papyrus with Harry Potter, thinking it could be some sort of fanfiction concept. But it's a very far - fetched idea.
Some people might have a very muddled view of different fictional universes. They could be so used to the idea of fanfiction and crossovers that they randomly throw together different popular franchises like 'Harry Potter', 'lucifer', and 'buffy' in a completely wrong way. Also, they might not have studied or appreciated the unique aspects of each work deeply enough to realize how wrong this statement is.
Perhaps it's the result of a really out - there fan theory. Some fans like to create extreme or unusual theories about their favorite characters. In this case, someone might have come up with the idea that Yugi Muto 'abused' fanfiction without any real foundation. It could also be a case of someone trying to stir up controversy for attention.
Some people might come up with such out - of - place ideas just for shock value. They want to create something that is so different from the original that it grabs people's attention, even if it's in a negative or inappropriate way.
Another way is to read a lot of different books, watch movies, or listen to music. Inspiration can come from various sources. Sometimes a random thought or a dream can also turn into a story idea.
Well, you can start by observing your daily life. The little things, like a funny conversation or a unique place you visit, could spark an idea. Also, reading a lot of different books and genres can inspire you.