Another possibility is that Blackbeard, who originally had the dark dark fruit, could become the main antagonist. He might feel that Luffy has stolen his power and seek revenge. Blackbeard could team up with other powerful pirates or even make a deal with the World Government to get Luffy. He would use his knowledge of the fruit's powers to try and defeat Luffy in a battle of shadow and darkness.
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.
In such a fanfiction, perhaps Luffy's character would change drastically. The dark dark fruit might give him new powers like controlling shadows more intensely than Blackbeard. He could use it to create shadow clones of his crew for strategic battles, adding a new and darker dimension to his fighting style.
The Dark Dark Fruit could enhance Luffy's physical strength in a different way. Instead of just relying on his rubber body, the fruit's power could give him a more menacing presence in battle. He could use the darkness to absorb attacks and then redirect that energy, making his counter - attacks even more powerful. For instance, if an enemy throws a punch, Luffy could use the dark power to suck in the force of the punch and then send it back double - strength.
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.
One interesting concept could be Luffy having to deal with the corrupting influence of the Dark Dark Fruit. Maybe he struggles to maintain his usual carefree and positive attitude while using its powers. For example, the power might make him more withdrawn at first as he tries to figure out how to control it without losing his sense of self.
However, in the world of One Piece, it's been said that eating two devil fruits would cause a person to explode. But in fanfiction, this rule might be bent. Another consequence could be that Luffy's personality changes. The new devil fruit power might come with some sort of consciousness or influence that alters his decision - making or how he views the world. He might become more aggressive or more cautious depending on the nature of the new power.
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.
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.