Perhaps a group of anti - technology extremists could be the main antagonists in an Ultron fanfic. They see Ultron as the symbol of all that is wrong with technology and are determined to take him down, no matter the cost. They might use their own advanced, but more rudimentary, technology to fight against Ultron. This would create an interesting contrast between Ultron's high - tech abilities and their more basic, yet effective, methods.
The main antagonist could be a jealous acquaintance of Cosimo. This person has always been envious of Cosimo's status and charm. When he sees Cosimo falling for Contessina, he decides to sabotage their relationship. He starts by spreading malicious lies about Contessina, making Cosimo question her true nature.
An organization of rogue scientists who want to experiment on Pokemon could be the main antagonist. They might be trying to create super - powerful mutant Pokemon or find a way to control all Pokemon. If you're reborn as a Pokemon, they might capture you for their experiments. This could lead to a story where you have to escape from their labs and expose their evil plans to the rest of the Pokemon world.
A powerful, ancient sorcerer could be a great main antagonist. This sorcerer could have been locked away for centuries but has now broken free and wants to take over the universe from the Master of the Universe. He could have dark magic that the Master has never faced before.
The first queen could be a great antagonist. She might be jealous of the remarried empress's new position and try to undermine her at every turn.
A powerful rogue ninja who has discovered a way to control digital creatures could be the main antagonist. This ninja might see Impmon as a key to unlocking even greater power and would stop at nothing to capture him. He could use his knowledge of ninjutsu and his new - found ability to manipulate digital entities to create chaos in both the ninja world and potentially the digital world if he were to gain access to it through Impmon.
It could be a powerful sorcerer who wants to take over Crookshnks' homeland. This sorcerer has dark magic and sees Crookshnks as a threat to his plans, so he devises a scheme to eliminate him.
A rogue Cybertronian who has his own agenda and doesn't care about the Autobot - Decepticon conflict. This character could see Jack as an easy target or a means to an end, like stealing some valuable Earth technology that Jack has knowledge of.
Frieza could be a great antagonist. He has a history with Broly and would be interested in Bulma's technology. He might see Broly's new alliance with Bulma as a threat and try to break it up. Frieza could also want to use Bulma to create powerful weapons or devices for his own evil purposes.
It could be a rogue ninja who has mastered parkour to an extreme level. He uses his skills to steal important ninja artifacts and Naruto has to stop him. This antagonist would be very fast and agile, making it difficult for Naruto to catch him using normal ninja techniques.
Orochimaru could be a great antagonist. He might want to use Shenron's power for his own dark experiments. He could try to steal the dragon balls and manipulate the wishes to gain immortality or create an army of powerful, immortal ninja.