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.
Well, it could be that the Flamels are in danger due to Voldemort's return. Harry Potter, being the hero, steps in. He might enlist the help of his friends like Ron and Hermione. They could go on a quest to find a powerful magical artifact that can safeguard the Flamels. Harry's knowledge of the wizarding world and his past experiences fighting evil would be crucial in this story.
Obviously, Harry Potter would be one of the main characters. Since it's about him saving the Flamels. The Flamels themselves, Nicolas and Perenelle, would also be main characters as they are the ones being saved.
Well, perhaps Harry could use his magic to defend Wakanda from some dark force. Maybe Voldemort has an alliance with an enemy of Wakanda and Harry has to stop them. He could use spells like Protego Maxima to shield Wakanda's borders and use the Patronus charm to fend off the Dementors that the villains might send.
One possible plot could be that a powerful dark wizard has set his sights on the Cullens for some reason, maybe to use their vampire powers for his own evil deeds. Harry, being the hero he is, senses this danger through his connection to the magical world. He then teams up with the Cullens, teaching them some basic wizarding defenses like Shield Charms. He also uses his own powerful magic, such as the Killing Curse in a defensive way if necessary, to save the Cullens from the clutches of this dark wizard.
It might be a character from a different fairy - tale world that has been magically transported into the Harry Potter universe. Let's say it's Snow White. She retains her beauty and kindness but is now in a world full of wizards and witches. Voldemort or one of his followers wants to use her for some evil purpose and Harry Potter has to save her. He would be quite confused at first but then rises to the occasion.
Well, perhaps there is a dark force that is after Maximoff and it's affecting different universes. Harry Potter, being the hero he is, senses this disturbance in the magical fabric. He travels to Maximoff's realm using his Patronus as a guide. Once there, he uses his knowledge of defensive magic, like the Shield Charm and Stunning Spells, to protect Maximoff and save him from the impending doom.
The plot could start with Harry Potter noticing strange magical anomalies. These anomalies turn out to be related to Maximoff's world. Harry decides to investigate and finds Maximoff in a dangerous situation. He then uses his magical abilities to rescue him.
Well, perhaps Harry stumbles upon a goblin in a dangerous situation, like being trapped by dark wizards who want to steal the goblin's magic artifacts. Harry uses his skills and spells to fend off the bad guys and rescue the goblin.
Voldemort would still be a likely villain. He is the main antagonist in the original series and his threat to Harry is always present. Even if Harry is trying to save himself, Voldemort's power and his desire to kill Harry would make him a formidable foe.