Yes, there are likely fanfictions out there that combine elements of Harry Potter and the Battle of Helms Deep. Fanfiction writers often like to mash up different fictional universes. You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own.
An interesting plot might involve a prophecy that links the fate of both worlds. The prophecy states that only by combining the power of the Elder Wand and the sword of Aragorn at Helms Deep can the ultimate evil be defeated. Harry, along with Hermione and Ron, journey to Middle - earth. They face many challenges on the way. At Helms Deep, they have to work together with the people there. Harry uses his magic to enhance the strength of the defenders, while the others from Middle - earth contribute their combat skills. There could be a sub - plot where some of the characters from both worlds develop romantic interests in each other, adding an extra layer of complexity to the story.
One possible plot could be that Harry uses his magic in ways that are unexpected in the Middle - earth setting of Helm's Deep. Maybe he uses spells like 'Protego Maxima' to create a shield around the fortress, protecting it from the incoming orcs. He could also use the 'Accio' charm to bring weapons to the defenders when they are in need.
Yes, there might be. The 'Harry Potter' and 'One Piece' (where Marineford is from) fanfic communities are quite large and creative. There could be a crossover where Harry Potter somehow saves Ace at Marineford. You can check fanfic platforms like Archive of Our Own or FanFiction.net to see if such a story exists.
In some fanfictions, Harry might use his magic skills. For example, he could use spells to protect Naruto from a powerful enemy. Maybe he casts a shield charm to block an attack that Naruto couldn't avoid on his own.
It could be some dark wizards who want to use the centaur's parts for potions. They might be following some ancient, dark magic recipe that requires centaur hair or hooves. So they hunt down the centaur, but Harry gets in their way.
In this fanfiction, perhaps the centaur is in a situation where it's about to be sacrificed in some dark ritual. Harry discovers this and races against time to stop it. He calls upon his friends Hermione and Ron for help. Hermione uses her vast knowledge of magic to figure out how to break the dark magic circle, while Ron provides some much - needed comic relief during the tense situation. Harry, of course, is the one who ultimately uses his powerful Patronus charm to dispel the dark forces and save the centaur.
Zoe could be a new student at Hogwarts who gets into trouble. Maybe she's a first - year who wandered into the Forbidden Forest by accident and Harry has to save her from the dangerous creatures there.
Well, perhaps Zoe is in danger from dark wizards. Harry, with his skills and bravery, could use spells like Expelliarmus to disarm the villains threatening Zoe. He might also use his invisibility cloak to sneak up on them.