Lex Luthor might also be a candidate. He could be looking to expand his power across different universes and sees the combined strength of Wonder Woman and Spider - Man as a threat. He would use his vast intellect and resources to try and take them down, perhaps by creating some sort of super - weapon that targets their respective powers.
The Green Goblin could be a likely villain. His chaotic nature and powerful glider - based attacks would pose a great threat to both Wonder Woman and Spider - Man. He could be after some sort of powerful artifact that Wonder Woman has knowledge of, and Spider - Man gets involved trying to stop him from getting it.
Circe is another option. She has the power to transform people and cause chaos. In the fanfiction, she could be jealous of Wonder Woman's beauty and power and Percy's connection to the gods. She might try to turn them against each other by using her magic on them or on those around them, causing misunderstandings and battles.
Voldemort could be a great villain. Imagine him getting his hands on some of Spider - Man's technology. He could use it to enhance his Death Eaters' powers or even create new, more powerful dark magic. Spider - Man and Harry would have to put aside their differences in fighting styles and worlds to stop him.
Ares could be a great antagonist. He might see the union of Black Widow and Wonder Woman as a threat to his plans of chaos and war. His god - like powers would pose a significant challenge to the two heroines.
It could be an evil Sith Lord who discovers the existence of the Naruto universe and wants to conquer it. He sees Naruto and Ahsoka's alliance as a threat and tries to stop them. This Sith Lord might use his dark side powers to corrupt some of the characters from the Naruto universe and turn them against Naruto and Ahsoka.
It could be Orochimaru. His thirst for power and his willingness to experiment on others would put him at odds with both Naruto and Deadpool. Orochimaru might see Deadpool's regenerative abilities as something to study and Naruto as a threat to his plans. He could try to capture them and use their powers for his own dark purposes.
Maybe a new villain created specifically for the fanfiction. A tech - savvy criminal who wants to steal the secrets of both Stark Industries and Wakanda's technology. This villain could hack into Stark's systems and also try to infiltrate Wakanda. He would be a threat to both Tony and Black Panther as he aims to use their combined knowledge for his own evil purposes.
The Joker could be a great villain. Imagine him getting his hands on some ghostly technology or artifacts from Danny's world. He would use it in his own chaotic and unpredictable way, causing mayhem in Gotham and also attracting Danny's attention. Danny and Batman would have to work together to stop the Joker's mad schemes.
Another possible villain could be Fenrir Greyback. He is a werewolf in the Harry Potter world, and Wolverine also has some werewolf - like qualities. Fenrir might see Wolverine as a rival. He could team up with some dark wizards to try and take down Wolverine and Harry Potter. This would create a complex web of battles and power struggles between different factions in the fanfiction.
Circe could be a great villain. She has powerful magic and could create all sorts of illusions and obstacles for Wonder Woman and Hawkgirl. She might try to turn them against each other or use their powers for her own dark purposes.
Doc Ock is a likely villain in Spider - Man angry fanfiction. His mechanical arms and his intelligence make him a formidable foe. His evil plans usually put the city in danger, and Spider - Man would be extremely angry trying to stop him. Also, Electro could be a villain in such fanfictions. His ability to control electricity can cause a lot of destruction, and Spider - Man would be angry at his disregard for human life while causing chaos.