Dark Matter is often a popular antagonist in dark Kirby fanfiction. It could be presented in a more powerful and uncontrollable form, corrupting everything it touches in Kirby's world. It might also have the ability to possess other characters, turning them against Kirby.
Some fanfictions might introduce an entirely new antagonist, like a powerful sorcerer from a different dimension. This sorcerer could be jealous of Kirby's powers and wants to steal them for his own dark purposes. He would use his magic to create all kinds of obstacles and traps for Kirby, making the hero's journey extremely difficult.
Well, one common antagonist is Reactron. His radioactive powers can be a match for Supergirl, and in a dark supergirl fanfiction, he could be even more of a threat as she might be dealing with her own internal struggles. Another could be the Cyborg Superman. His combination of kryptonian and cybernetic powers makes him extremely powerful. He could be a great antagonist for a dark Supergirl as he might try to use her for his own purposes or simply see her as a rival. Also, the Phantom Zone criminals can be antagonists. They are escaped convicts from Krypton's most dangerous prison, and a dark Supergirl might have to deal with them while also trying to come to terms with her own dark side.
Voldemort or his remaining Death Eaters are often common antagonists. Even after his defeat in the original story, in dark fanfiction, they could come back in some form to target Harry and Ginny. For example, a Death Eater who escaped justice might seek revenge on Harry by harming Ginny.
There could be new original antagonists created specifically for the fanfiction. These might be dark entities that are related to the source of Dark Sonic's power, trying to either control him or use his power to spread chaos in Sonic's world.
In some cases, corrupt Ministry of Magic officials can be antagonists in 'Harry Potter Dark Business Fanfiction'. They could be involved in covering up or even profiting from the dark business. For example, they might turn a blind eye to the illegal trading of dangerous magical creatures in exchange for bribes. This would make them enemies of Harry Potter and those who strive for a just and clean wizarding world.
Another theme might be friendship. Even in a dark future scenario, the characters could come together and use their friendship as a strength to overcome the challenges posed by Sombra. In many MLP stories, friendship is a central theme, and it could be incorporated into this fanfiction concept as well. For example, Kirby might form an unlikely friendship with the ponies and together they face Sombra. Also, themes of redemption could be present. Maybe Sombra has a chance to change his ways in this dark future storyline, adding an interesting layer to the plot.
One possible main antagonist could be an ancient evil that has been awakened by the discovery of the Dark Excalibur. This entity might be seeking to use Sonic as a pawn to spread chaos and destruction. For example, it could be a long - forgotten dark wizard who once forged the Excalibur for evil purposes.
The main antagonists might be a new breed of ninja created by some dark jutsu or experiment within the 'dark empire' fanfiction. These new ninjas could have powers that are a threat to Naruto and his friends, and they are bent on spreading chaos and darkness throughout the Naruto world. They might have no moral compass and be solely focused on domination.
Often, ancient and powerful demons or dark gods can be antagonists. They might see Naruto's newfound godhood as a threat to their own power or plans. For example, a demon that has been sealed away for centuries could be awakened by Naruto's ascension and seek to destroy him.
The other shinobi villages might also be antagonists. If Naruto's village is seen as a threat because of his power, they could target him. This would make Naruto angry as he's being attacked for no good reason other than fear. Their actions against him and his village could lead him to a darker state as he has to fight back against these external threats.
One potential antagonist could be an ancient evil that is awakened by Ichigo's newfound power. This entity might see Ichigo as a threat or as a means to achieve its own dark goals.