An ancient demon that was sealed away long ago but has now been released. This demon is much more powerful than Sebastian and has a vendetta against the Phantomhive family for some past transgression. It would pose a huge threat to Ciel and Sebastian, forcing them to seek help from unexpected sources.
A powerful cult that worships a dark entity. They are trying to bring about the end of the world as we know it and Ciel and Sebastian get in their way. The cult members have strange and dangerous powers, and they are very secretive and well - organized, making them a formidable enemy.
A group of rogue reapers. They don't follow the rules of the Death Gods and are causing chaos in the afterlife and the living world. Their actions start to affect Ciel's plans and he has to deal with them, which is difficult as they have unique powers related to death and time.
Alois could be a main antagonist. In a high school setting, he might be the jealous rival of Ciel. He could try to undermine Ciel's position as student council president or spread rumors about him. His butler, Claude, would be helping him with his devious plans.
It could be a new female character with black hair and red eyes. She could be a witch who has a connection to Sebastian's past. Maybe she was once his ally but now has her own agenda in the 'Black Butler' world.
The established nobles who fear losing their power could be antagonists. They might try to plot against the new king to keep their old ways and privileges. Another potential antagonist could be a powerful and jealous rival who believes he should be the king instead.
One idea could be that Ciel has finally achieved his revenge, but now he finds himself lost without a purpose. He then discovers a new threat to the underworld that he feels compelled to fight against. This could involve new characters and a whole new set of mysteries.
An alien race that discovers Lyoko and decides to invade it and use it as a base to conquer the real world. They would have completely different powers and technologies compared to the threats the Lyoko Warriors have faced before, forcing the heroes to think outside the box.
The Death Eaters who still hold onto the old pure - blood supremacist views could be main antagonists. They might see the Heir of Arcturus Black as a threat to their ideology, especially if the Heir is trying to change the Black family's ways.
She could be a long - lost relative of Ciel. Maybe an aunt or a sister that was thought to be dead. Her appearance in the garden could be a sign of something bigger about to happen in the Phantomhive family.
Well, if Ciel watches the future in a Black Butler fanfiction, it depends on how the author writes it. One possibility is that he sees a future where his family is avenged but at a great cost, like losing his soul to Sebastian earlier than expected. Another is that he sees a future where his enemies have new allies and he has to find different ways to defeat them.
A young witch from a rival coven could be added. She might be initially antagonistic towards Ciel and Sebastian but later form an uneasy alliance when a greater threat emerges. She could bring in new magical elements and challenges to the story.
Sebastian Michaelis could also be a main character in some fanfics. His demonic nature and loyalty to Ciel can create interesting interactions with Vintaker. Their different motives and abilities can lead to intense confrontations or unexpected alliances in the fanfic.