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.
One possible plotline is that Harry is magically transported to the Naruto world and discovers Naruto is in danger from a new, powerful enemy. Harry then uses his magic to fight alongside Naruto and his friends, saving Naruto in the process.
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.
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.
The obvious antagonists would be the Death Eaters. They are Voldemort's followers and are often causing trouble in the wizarding world. In this fanfiction, they might be the ones who have put Cedric in danger, either to lure Harry or for some other dark purpose.
The fanfiction could show that Harry discovers a secret passage that leads to where Cedric is held captive. He has to face various magical traps and challenges on the way. Once he reaches Cedric, he uses his Gryffindor courage and his knowledge of magic, like the Shield Charm to protect Cedric from incoming spells, and then they escape together, with Harry being the hero who saves Cedric.