Perhaps an evil scientist who was the one responsible for creating the energy beam in the first place. He or she might see Carol as an unexpected variable in their experiment and wants to eliminate her. They could also be trying to use Carol's new - found power for their own dark purposes.
The main antagonist could be a powerful alien race. They might have sent the energy beam to Carol's planet as a way to test its power on the inhabitants. When Carol absorbs it and becomes powerful, they see her as a danger to their plans of conquest. So they send their best warriors or use their advanced technology to try and stop Carol.
It could be that Carol is in a situation where she has no choice but to absorb the energy beam. This might be part of an experiment gone wrong or an attack on her that she turns to her advantage. After absorbing it, she might struggle to control the new energy within her and has to go on a journey to master it. She could meet other characters who help or hinder her in this process.
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.
It could be another alchemist who wants to use this new energy for evil purposes. This alchemist might see Edward as a threat to his plans and constantly try to stop him from learning more about the energy.
In 'A Christmas Carol', Ebenezer Scrooge can be seen as the antagonist initially. His selfishness and lack of compassion for those around him create conflicts and challenges. However, as the story progresses, he undergoes a transformation and is no longer the antagonist.
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.
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.