To combine them well, consider the lore of both. In the 'Harry Potter' world, there are different magical creatures and beings. Ben Drowned could be seen as a new kind of 'creature' that has entered the wizarding world. He could bring with him new forms of magic that the wizards are unfamiliar with. The 'Harry Potter' characters might be curious about him, but also cautious. And for Ben Drowned, the Hogwarts environment with its rules and traditions could be both fascinating and frustrating. His character could disrupt the normal order of things at Hogwarts, which would create a lot of story opportunities. For example, he might accidentally break some magical laws that the characters then have to cover up or deal with in a different way.
Well, one way is to make Ben Drowned a part of the Hogwarts mystery. Maybe he's linked to a secret chamber that has been long forgotten. The 'Harry Potter' characters then have to figure out how he's related to it while also dealing with his otherworldly nature. His presence can add a new layer of danger and mystery to the Hogwarts environment. Also, his powers could be a contrast to the magic that the students are used to. For instance, if the students use wands for magic, Ben Drowned might have magic that comes from within him in a more instinctive way. This can lead to interesting interactions and power struggles within the fanfiction.
There might be a fanfiction where the characters in Harry Potter interact with Ben Drowned in a Hogwarts setting. For example, Harry and his friends could initially be suspicious of Ben Drowned's strange behavior and appearance. But as the story progresses, they realize that he may be a key to solving a mystery in the wizarding world, like a new dark magic that has seeped into the school and only Ben Drowned's unique perspective can help them figure it out.
One way is to create a magical item. For example, a lemon - shaped amulet that has the powers of fairies and is given to Harry Potter. It could grant him abilities like flying faster on his broom or being able to understand the language of magical creatures. Another way is to have a fairy character who is obsessed with lemons and befriends Harry. They go on adventures together in the wizarding world, using the fairy's lemon - related magic.
Magic spells from Harry Potter can be given Magic the Gathering - like mechanics. For example, the 'Expecto Patronum' spell could cost a certain amount of mana (like white mana for its purity aspect) and be used to protect against dark forces, which in the fanfiction could be related to certain types of Magic the Gathering creatures like demons.
A creative combination could be that the Central Perk coffee shop is secretly a wizarding hangout in the Muggle world. The Friends gang unknowingly serves magical drinks to wizards and witches in disguise. There could be a plot where the regulars at Central Perk start to notice strange things happening, like people disappearing into thin air or cups refilling themselves. And the Friends characters have to figure out what's going on with their new 'weird' customers.
The settings can also be combined. Picture a scenario where the Forbidden Forest in Hogwarts is used as a training ground by Kakashi for the students. He could set up ninja - like obstacle courses among the magical trees and dangerous creatures. This would bring a whole new dimension to the Hogwarts experience.
Another way is to create a crossover where the worlds of Naruto (where the Mangekyou Sharingan comes from) and Harry Potter collide. In this scenario, the rules of both universes would have to be adjusted. For instance, the magic in Harry Potter could interact with the chakra system of Naruto. A character with the Mangekyou Sharingan could use it to detect hidden magical spells or even use their Sharingan - based abilities in magical duels, adding a new dimension to the battles in the Harry Potter world.
The worlds can be combined by having characters cross over. For instance, Superman could be called to Hogwarts to help deal with a dark wizard who has powers that are beyond the normal magical abilities. His invulnerability would be a great asset in battles against dark magic.
One way is through magical items. For instance, a magical object from the Harry Potter world like the Philosopher's Stone could end up in Tinkerbell's world and she has to protect it from the dark fairies. Or vice versa, a fairy magic item could find its way into Hogwarts and cause a stir.
A prophecy could be written in the fanfiction that ties their fates together. So, they are forced to interact and rely on each other's skills. Kalona's strength and Harry's knowledge of magic could complement each other in fulfilling the prophecy.
They can be combined by making the Black Mamba a part of the magical creatures in the 'Harry Potter' universe. For example, it could be a new kind of Animagus form.