Who could be the main antagonist in a 'Harry Potter saves Ace from One Piece' fanfic?

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.

It could be a powerful sorcerer from a magic - based crossover world. This sorcerer sees Ace's power as a threat to his own rule and plans to either capture Ace and use his power for his own evil deeds or simply destroy him.

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.

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.

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.

It could be an ancient dark magic artifact. The artifact has a hold on Bellatrix, and in a way, it's the 'enemy' that Harry has to defeat to save her. The artifact might be trying to use Bellatrix to carry out some dark plan, like opening a portal to a world full of dark creatures. Harry has to figure out how to break the connection between Bellatrix and the artifact while also fending off the artifact's attacks.

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.

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.

The main antagonist could be a powerful necromancer who combines the dark arts of Skyrim with some elements of Voldemort - like magic. He could be raising an army of the undead and causing chaos across Skyrim, which Harry has to stop.