Perhaps a new, original villain created specifically for the fanfiction. This villain could be a powerful sorcerer who wants to steal the powers of Wonder Woman and Power Girl to increase their own magic. They could use illusions and dark spells to try and defeat the two heroes.
Darkseid could be a main antagonist. His thirst for power and control could put him in direct conflict with Wonder Woman and Power Girl. He might see their combined strength as a threat to his plans of universal domination.
Often, it could be some sort of super - powered being from a different dimension. Maybe a powerful wizard who wants to take over both their universes.
Darkseid can also be a common antagonist. His goal of universal domination means that he would see Superman and Power Girl as threats to his plans, and he would send his minions or come himself to fight them.
Lex Luthor could be a main antagonist. He might see their union as a threat to his power and schemes, and try to break them apart or use their relationship against them.
Another storyline might involve a misunderstanding at first. Wonder Woman and Power Girl could have different approaches to justice and initially clash. But as the story progresses, they learn from each other and end up becoming a great team. For example, Wonder Woman's more diplomatic style and Power Girl's more brash, direct approach could create some initial tension. However, when they see the value in each other's methods, they are able to work together more effectively against common foes.
Doctor Psycho is a significant antagonist in these stories. His mental powers allow him to manipulate situations and people, and he frequently uses them to target Wonder Woman and put her in harm's way. Another one could be Circe. As a powerful sorceress, she can cast all kinds of spells on Wonder Woman, whether it's to weaken her, trap her, or make her lose her way, thus putting her in peril.
Cheetah could also be a main antagonist in these stories. She is one of Wonder Woman's most well - known foes. If she manages to capture Wonder Woman, it could be part of her scheme to prove herself as the superior fighter. Cheetah's enhanced speed and strength make her a formidable captor.
Yes, there are. For example, some fanfictions might focus on their team - up adventures, where they fight against common foes in the DC universe. They could combine their unique powers and skills to take on powerful villains.
The main antagonists could be the Titans who see Percy as a threat because of his combined powers. They might try to stop him from fully realizing his potential as the son of Wonder Woman.
Darkseid is another potential foe. He could be interested in the son's unique heritage, wanting to either corrupt him or use his powers for his own evil purposes. Given Darkseid's power, the son would have to rely on both his Batman - like wits and Wonder Woman - like strength to oppose him.
You can try some popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of DC Comics - related fanfictions, and you're likely to find some great stories about Wonder Woman and Power Girl there.