Well, 'Kingsman at Hogwarts' is a fun one. In this fanfiction, the Kingsman organization decides to send an agent to Hogwarts to investigate some strange magical occurrences that might have an impact on the Muggle world too. It has a lot of action scenes where the agent uses both magic (learnt while at Hogwarts) and his Kingsman gadgets.
Yes, there might be some out there. However, it's not a very common combination. You'd have to search through various fanfiction platforms. Some might explore the idea of Merlin - like characters in the Kingsman universe interacting with the magical world of Harry Potter in unique ways, but it would be a very niche sub - genre within fanfiction.
A really interesting 'harry potter and kingsman fanfiction' might involve a character from Harry Potter, like Hermione, joining the Kingsman organization. She could use her intelligence and magic to help the Kingsman in their missions. This would bring a different perspective to the Kingsman world as they deal with someone who has magical abilities and a different set of values from their normal recruits.
Another theme is the idea of using different skills. In these fanfictions, you often see Harry Potter characters using their magic in conjunction with the Kingsman's combat and espionage skills. It's about how these two different skill sets can be combined to achieve a common goal, like saving the world from a new, combined threat.
A really interesting crossover is when the magic of Harry Potter is used to enhance the gadgets in Kingsman. For example, in some fanfics, they create wands that can also function as Kingsman umbrellas, with the ability to both shoot spells and act as a shield or a parachute. It's a cool blend of the two worlds' technologies.
One great 'Ninja Harry Potter' fanfiction is 'Harry Potter: The Ninja's Path'. It combines the magic of the Harry Potter world with ninja skills in an exciting way. The story follows Harry as he discovers a hidden ninja heritage and uses these new skills to face Voldemort.
I really liked 'Harry Potter and the Hidden Animagus'. In this fanfic, Harry discovers he has a unique connection to Animagus magic that was hidden from him. It has an interesting plot that keeps you hooked from start to finish. Also, 'The Animagus Bond' is a great read. It explores the relationships between Animagi and their animal forms in a really engaging way.
I really like 'The Christmas Miracle at Hogwarts'. In this fanfic, there's a problem with the magic that makes Christmas possible at the school. Harry and his friends have to figure out how to save Christmas. It's a heartwarming story that shows their friendship and determination. It also has some really cool descriptions of the Christmas spells and how they work.
One great Loki and Harry Potter fanfiction is 'The Mischief in Hogwarts'. It has Loki somehow ending up in the wizarding world and causing all sorts of chaos and hilarity. He teams up with Harry Potter and they face some dark wizards together. It's a really fun read with lots of action and the unique blend of the two universes.
One great 'Hitman Harry Potter' fanfiction is 'The Hitman and the Boy Who Lived'. It combines the stealth and assassin - like skills from the Hitman concept with Harry Potter's magical world in an exciting way. The story often has Harry using his wits and some unexpected combat skills to deal with threats in Hogwarts that are more complex than just dark magic.
One great merman Harry Potter fanfiction is 'The Merman's Magic'. It has an interesting plot where Harry discovers he has a connection to the merman world. The author does a great job of blending the magical elements of the Potterverse with the unique lore of mermaids and mermen.