It could be a dark wizard. Dark wizards are often associated with using dark arrows as weapons in fanfiction. They might have a vendetta against Chris for some reason, like he unknowingly disrupted one of their dark magic experiments.
One possible plot twist could be that the dark arrow has some sort of magic that starts to transform Chris into a dark being against his will. As he struggles with this new dark side, his friends or family have to find a way to reverse the process.
A powerful vampire - witch hybrid could be a great antagonist. This character could combine the strength and immortality of a vampire with the magical abilities of a witch. They might envy Bella's connection to both the vampire and human worlds, and also be jealous of Chris's powerful magic lineage. Their goal could be to create a world where vampires and witches rule over humans.
Cinderella's fairy godmother could also turn out to be the antagonist. She might have given Cinderella false hopes and used her to fulfill some self - serving prophecy. Maybe she has her own dark secrets and is manipulating Cinderella all along.
Perhaps a group of dark side cultists could be the main antagonists. They idolize the power of the dark side and see Ahsoka as a prime candidate to join their ranks. When she refuses at first, they start to hunt her down, but in the process, they unknowingly push her closer to the dark side, creating a complex dynamic where they are both the pursuers and the inadvertent cause of her fall to the dark side.
The Green Goblin could be a great main antagonist in a dark fanfiction. He could be even more deranged and have a more elaborate plan to destroy Spider - Man, not just physically but also mentally by targeting his relationships.
It could also be a magical being from another dimension. This being might have caused Chris's amnesia on purpose in order to study the Charmed Ones' reactions. They could be interested in the power dynamics within the family and how they deal with a member who has lost their memories. Their actions could range from simply observing to actively trying to disrupt the family's harmony.
The World Government could become the main antagonist. They would see Luffy's new power as a threat, especially if he starts using it in ways that oppose their rule. They might send out their strongest agents, like the Admirals, to take Luffy down. The Admirals could have developed new techniques to deal with the dark dark fruit powers, making them a formidable foe for Luffy.
It could be an unknown ninja who has been secretly studying dark energon for years. He wants to use it to take over the ninja world, believing that the power of dark energon will make him invincible.
Well, obviously Dark Gaia itself could be the main antagonist. It has the power to cause chaos and destruction, and Sonic would have to stop it from wreaking havoc on the world.
It could be a powerful sorcerer. In the Dark Skies world, a sorcerer with dark magic could oppose Jesse. This sorcerer might be after the same magical artifacts as Jesse or simply wants to rule the Dark Skies with an iron fist.