It could be another character who is jealous of Lucy's new powers. Maybe a classmate or a rival in her community who wants those powers for themselves and will do anything to take them from Lucy.
Maybe Lucy discovers an ancient artifact that gives her new powers. For example, she could find a magic amulet that allows her to control the elements. Then she has to learn how to use these powers to save her town from a mysterious threat.
Maybe the main antagonist is an old enemy of Robin's who becomes jealous of his new powers. This enemy then devises a plan to steal Robin's powers or at least neutralize them so that he can regain his former status as the top threat. He could be a powerful martial artist or a criminal mastermind with a large network of henchmen.
A new character could be introduced as the main antagonist. Maybe a powerful alien who sees Kim's new powers as a threat to his plan of taking over the earth. This alien could have his own set of powers that are quite different from Kim's, like the ability to control minds. Kim would have to not only learn to use her new powers but also find a way to defeat an enemy with such a dangerous ability.
A dark entity from another dimension could be the main antagonist. It senses Anna's new powers and wants to use them to open a portal between dimensions so that it can bring its army of monsters into Anna's world. So it constantly hunts Anna down, trying to either force her to use her powers for its own evil plan or simply kill her and take the powers for itself.
It could be another person who also has some sort of mind - controlling abilities and wants to use the boy's powers for their own evil plans. This antagonist might try to trick the boy into using his powers in the wrong way.
Perhaps he gets a power related to mind control. This would allow him to influence the actions of his enemies or even make them fight among themselves. He could also gain a power that lets him absorb the powers of his defeated foes. So, every time he defeats an enemy, he becomes stronger by adding their powers to his own arsenal.
Another possibility is that one of Leo's classmates becomes jealous of his powers. This classmate could start spreading rumors about Leo, trying to turn everyone against him. Eventually, the jealousy could turn into something more sinister, and the classmate could team up with some bad elements to try and take away Leo's powers.
It could be an ancient and powerful wizard. Maybe the wizard sees potential in Beast Boy and decides to grant him new powers to help him in his fight against evil.
One possible new power could be the ability to control electricity. He could use it to stun enemies, power up machinery, or even create electromagnetic fields to deflect attacks. This would add a whole new dimension to his combat style and problem - solving skills.
It could be another dark guild. They might be using Gray as a pawn to kidnap Lucy, perhaps because they know of her celestial spirit magic and want to use it for their own evil purposes. Gray may not be fully aware of their manipulation.