It's highly inappropriate. The relationship between Umbridge and Harry Potter is one of oppressor and oppressed in the original story. Adding such a violent and inappropriate act like spanking in a fanfic not only distorts the characters' relationship but also may have a negative impact on fans, especially younger fans who look up to the series for positive values and role models.
The novelty. Usually in the original series, we have a set idea of what professors are like. In a 'a different professor Harry Potter fanfic', they can be completely different in terms of personality. For example, a super shy professor who is an expert in Dark Arts but is always afraid to show off his knowledge.
A really cool fanfic idea could be that Professor Sinistra discovers Harry Potter has a natural aptitude for a very rare form of astronomy - based magic. She then decides to take him under her wing privately, away from the prying eyes of other professors and students. This could lead to a series of adventures where Harry learns this unique magic and uses it to solve problems at Hogwarts.
In Harry Potter fanfiction, a male professor can play various roles. For example, he could be a mentor figure like Dumbledore, guiding the young wizards. He might also be a villainous professor, creating obstacles for the main characters. Some fanfictions might explore a male professor's backstory, his relationship with other characters in the wizarding world, or his own magical research and discoveries.
One popular 'a different professor Harry Potter fanfic' could be 'The Unconventional Professor at Hogwarts'. It might feature a professor with a unique teaching style or a different backstory that adds an interesting twist to the Hogwarts experience.
Well, in this fanfiction, Harry might be a professor because of his extensive knowledge and experience in magic. He could be teaching younger students and sharing his wisdom.
The Joker as a professor in Harry Potter fanfiction? That's a wild idea. He could be the professor of Dark Arts in a very different way. Instead of the traditional Dark Arts that the wizarding world knows, he would introduce his own form of 'dark art' which is more about psychological torture and mind games. His classes would be full of his maniacal laughter and unexpected challenges. Students would have to constantly be on their toes, not knowing what he'll do next. He might even use his students in his elaborate schemes against the other professors or the Ministry of Magic.
Yes, in some fanfictions Harry Potter becomes a professor. For example, in certain stories set after the main series, he might return to Hogwarts to teach Defense Against the Dark Arts, passing on his extensive knowledge and experiences from his own battles with the Dark Arts.
One great Naruto and Harry Potter crossover fanfiction could be where Naruto is somehow transported to the wizarding world and meets a Professor Potter. In this kind of story, Naruto might use his ninja skills in Hogwarts, like using his chakra to perform amazing feats that baffle the wizards. He could team up with Professor Potter to fight against dark forces that are threatening both the ninja world and the wizarding world.