A mysterious figure from the bad gal's past that she doesn't even remember clearly. This antagonist could have a deep - seated grudge against her and is now emerging to make her life miserable, and as the story unfolds, the bad gal gradually discovers who this person is and why they are after her.
It could be an old acquaintance who feels betrayed by the bad gal. Maybe someone she double - crossed in her previous adventures and now is back for revenge.
Another plot could be the bad gal trying to reform but her past keeps coming back to haunt her. Her old allies turn on her because they don't trust her new good - girl act, and she has to prove herself while also dealing with her inner demons.
It could be a new character, like a bully at school who doesn't like Ron's new bad boy attitude and constantly tries to pick fights with him.
It could be the Green Goblin. In the sequel, he could return more powerful and with a new plan. Maybe he has found a way to enhance his strength and insanity using some new Oscorp technology. He could target Spider - Man not only for revenge but also to prove his superiority over the web - slinger.
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.
Another option is that Metal Sonic could be the main antagonist. He might see Amy Rose's new bad side as an opportunity to create chaos and undermine Sonic. Metal Sonic could manipulate Amy Rose further into doing more evil deeds, all while trying to frame Sonic for her actions.
A rival medicine cat from another Clan might be the main antagonist. They could see Alderheart as a threat to their own status or the well - being of their Clan. So they might use dark magic or some underhanded tactics to bring about his death.
Vilgax could be a great antagonist. He could hack into the gaming systems to try and defeat Ben in a virtual world where the rules are different. He might use his advanced technology to create digital versions of his minions to attack Ben.
The Organization in the Claymore world could be the main antagonist. They already have a lot of secrets and do some morally ambiguous things. If Naruto shows up and starts changing the status quo, like teaching Claymore warriors to be more independent, the Organization might see him as a threat and try to eliminate him.
It could be a new alien species that Ben has never encountered before. This species could be territorial and see Ben as a threat to their planet where he's stranded. They constantly try to drive him away or capture him, which makes it difficult for Ben to find a way to leave.
It could be a rogue ninja who was once a student of Orochimaru and has now developed his own dangerous jutsu. He wants to take over the ninja world and believes the old ways are holding everyone back.