It could be a former Auror who has turned to more shadowy ways of dealing with threats. After seeing the inefficiency of the Ministry's methods in some cases, they decide to take matters into their own hands and become a sort of assassin to target dark wizards.
It depends on your taste. If you like the idea of Harry Potter in a completely different and edgier role like an assassin, it could be interesting. However, some purist fans might not like this deviation from the original concept of the series.
One interesting plot could be that the main character, who has assassin skills, is secretly protecting Hogwarts from a new and unknown dark threat that the regular wizards can't detect easily. They use their unique combat skills along with magic to keep the school safe.
Yes, there are likely to be such fanfictions. The 'Harry Potter' universe has a vast and creative fanfiction community. Some fans might come up with the idea of Harry Potter being a Muggle assassin as a way to explore different storylines and character developments outside of the original canon. It could involve Harry using his magical skills in a Muggle - centered, assassin - like role, perhaps to protect Muggles from dark magic threats in a more covert way.
You can expect a whole new set of adventures. Instead of the usual Hogwarts - related challenges, Harry will be facing assassin - like missions. There might be more sneaking around, using magic for covert operations. And his relationships with other characters could also change. For example, his friends might be more in the dark about his new activities, adding a layer of mystery to the story.
Perhaps an international wizard who has been trained in both magic and martial arts in some far - flung magical school. This wizard - assassin has his own code of ethics and when he meets Harry, he decides to take him under his wing, believing that Harry can be a force for good in a different way than the traditional Hogwarts - trained wizards.
It could be a rogue Death Eater who has mastered a new form of invisibility spell. This Death Eater might be seeking revenge on those who opposed Voldemort.
No. Harry Potter is mainly known for its own magical world and themes of friendship, good vs. evil, etc. Turning Harry Potter into an assassin fanfiction goes against the original spirit of the series and is likely not popular among the mainstream Harry Potter fans.
Jiraiya could be an interesting assassin master. He already has a lot of knowledge and skills that he could pass on to Naruto in a more assassin - like way. His experience in espionage and combat would make him a great fit.
Perhaps in a fanfiction, Voldemort has made an alliance with a powerful villain from the Avengers' world and an ancient order of Assassins gone bad. Harry Potter, along with some of his Hogwarts friends, has to convince the Avengers to help them. They would then combine their different abilities. For example, Hermione could use her vast knowledge to figure out the weaknesses of their enemies, while Captain America could lead the group in battle strategy, and the Assassin could provide crucial intel on the secret workings of the enemy alliance.