Perhaps a new character, a dark sorcerer. This sorcerer might be jealous of Sonic's popularity and power. So, he casts a spell on Sonic that makes him act out of character. In this 'dark sonamy story', Amy has to face this sorcerer who is much more powerful than she is, but she's determined to save Sonic no matter what.
In a 'dark sonamy story', the antagonist could be an alternate version of Sonic from a parallel dimension. This evil Sonic has come to their world to cause destruction. He tries to corrupt the real Sonic, and Amy is the only one who realizes what's going on. She has to find a way to send this alternate Sonic back to his own dimension and save the real Sonic from his influence.
It could be an evil wizard who wants to use Sonic's speed and Amy's strong will for his own dark purposes.
It could be about Sonic and Amy getting into a really tough situation in a spooky, shadowy world. Maybe they face a powerful and evil force that tries to break their bond.
Well, a 'dark sonamy story' might involve Sonic being corrupted in some way and Amy having to struggle to save him. Maybe Sonic gets possessed by an evil force and starts to turn against his friends, and Amy, with her unwavering love for Sonic, refuses to give up on him and embarks on a dangerous journey to find a cure or a way to break the curse.
The plot could be that Sonic gets cursed and starts to have a split personality. One side is his normal self and the other is a dark and violent version. Amy is in love with Sonic's normal side but is scared of the dark side. She tries to find a way to break the curse while still loving Sonic through all the changes and the danger that the dark side poses to her.
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.
One possible dark Sonamy love story could be that Sonic, usually the hero full of light, gets corrupted by an evil force. Amy, still in love with him, tries to bring him back. But Sonic, in his corrupted state, keeps pushing her away, yet Amy refuses to give up, and her love becomes a glimmer of hope in the dark situation.
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 a future version of himself. If Dark Hinata travels back in time, his future self might see this as a threat to the timeline he has created and try to stop him. This would create an interesting internal conflict within the character.