A powerful magical creature that has a grudge against both Harry Potter and Ace. It might see Ace as an intruder in its territory in the Harry Potter universe and Harry as someone who is interfering. So it tries to stop Harry from saving Ace and becomes the main antagonist.
It could be a powerful dark wizard who has somehow made his way into the Avengers' universe. Maybe a wizard who was banished from the wizarding world in Harry's universe and has now joined forces with some of the Avengers' villains.
Well, perhaps in this fanfic, Ace is on a mission in a strange new world that combines elements of both One Piece and Harry Potter. He gets into a tight spot when facing some magical creatures that are beyond his normal combat abilities. Harry Potter, who is familiar with this magical world, swoops in. He casts spells to disable the creatures and rescues Ace. After that, they might team up to explore this new and dangerous world together for a while.
Well, in such a fanfic, perhaps Harry uses his magic skills like the Patronus charm to ward off any danger threatening Ace. Maybe he apparates into the scene just in time to pull Ace out of harm's way.
One challenge could be the unfamiliarity with the Fairy Tail world. Different rules of magic and different types of magical creatures might be new to Harry. Another is that the Fairy Tail characters themselves might be very powerful and have their own unique abilities that Harry has to figure out how to counter.
It could be a dark wizard who has a vendetta against goblins. This wizard might see goblins as a source of power or wealth and wants to enslave them. So when Harry tries to save the goblin, this dark wizard becomes the main obstacle.
Perhaps a dark wizard from the Harry Potter universe who has heard about the Cullens' unique vampire abilities and wants to harness that power for himself. He could be a follower of Voldemort who escaped the final battle and is now looking for new ways to gain power. So he targets the Cullens, thinking their powers can help him rule both the wizarding and vampire worlds, and Harry has to stop him.
It could be a dark wizard who has his eyes on Cintra's powerful magical artifacts. He wants to use them to gain more power and is willing to destroy Cintra to get them.
It could be a rogue Death Eater who has escaped from Azkaban. This Death Eater might be desperate to regain his power and believes that unicorn blood will give him the strength he needs. So he sets out to capture a unicorn in the Forbidden Forest, which is where Harry comes in to stop him.
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.
Maybe a new character could be created as the main antagonist. A dark wizard who has infiltrated the Capitol and is using the Hunger Games as a means to create chaos and destruction. He could be more powerful than Voldemort in some ways and would be a great challenge for Harry and Katniss to overcome.