Perhaps the main villain is an old enemy of Jaden and Zane who has found a way to gain more power in the Dark World. He has set traps for them in this new and dangerous realm. He could be using the Dark World's unique properties to his advantage, like creating illusions that make Jaden and Zane doubt themselves and their friendship. The villain's goal could be to not only defeat them but also to use their defeat to gain even more power and spread his evil influence across both the Dark World and their home world.
The plot could involve Jaden and Zane getting trapped in the Dark World. Maybe they have to fight against dark creatures to find their way back home. They could discover new powers within themselves during this harrowing adventure.
Well, clearly Aster and Zane are the main characters. Aster might be the more impulsive one, always ready to jump into action, while Zane could be the more cautious, thinking through every move.
The Joker could be a great main villain. His chaotic nature would pose a huge threat to Bruce's goddaughter. He might target her just to mess with Batman, knowing how much she means to him.
The main villain could be a powerful and wealthy business - animal who is involved in illegal activities on the side. They use their legitimate business as a front to cover up their crimes, such as smuggling rare and endangered species for profit. They have a network of hench - animals who do their bidding and are very difficult to catch because of their influence in Zootopia's high society.
It could be a new teacher at the school who has very strict and old - fashioned views. This teacher doesn't believe in the progressive teaching methods that Riley and Maya want to implement, and constantly tries to undermine them. For example, the teacher might try to get their classes cancelled or their teaching materials removed.
Perhaps a jealous classmate. This classmate was always in the spotlight before Jaden turned into a girl. But now, with Jaden getting all the attention because of this strange transformation, the classmate feels left out and decides to be the antagonist. They might try to sabotage Jaden's new friendships or chances to participate in school activities.
Cinderella herself could be the villain. In a dark take, she might be using her charm and the situation to her advantage in a malicious way. For instance, she could be faking her kindness to the prince just to get into the palace and then cause chaos.
It could be a mad scientist. He could have been studying the Avengers from afar in their own universe and when they arrive in our world, he sees an opportunity. He might try to steal their powers or technology to create his own super - army.
It could be Zane's own negative self - thoughts. Since he is depressed, his inner demons constantly tell him that he is not worthy or that things will never get better. This internal struggle can act as an antagonist to the relationship between Laurence and Zane as it tries to prevent Zane from accepting Laurence's help and love.
There might also be a mysterious character who has knowledge about the origin of Jaden's dragon deck. This person could show up at crucial moments to give Jaden some crucial information about his deck, like how to unlock its full potential or the history behind some of the dragon cards. This character would add an element of mystery and intrigue to the fanfiction.