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.
Perhaps a strange alien encounter gives him new powers. The alien could have some unique technology or biological trait that, when combined with Beast Boy's abilities, gives him new powers. For example, the alien's energy could mutate Beast Boy's shape - shifting ability, allowing him to do more than just transform into Earth animals. He could now transform into alien creatures with extraordinary abilities.
Well, he might become even more powerful and be able to transform into new and amazing creatures. Maybe he could gain the ability to fly in his animal forms, which would be really cool.
He might gain the power of enhanced healing. So not only can he transform into different animals, but if he gets injured during a fight or accident, he can heal himself much faster than before. This would make him even more of a formidable hero in the fanfic's story line.
His friends might be in awe of him at first. They would see him as even more powerful and might rely on him more during battles. For example, if he has new powers like controlling the elements, they could use his abilities to their advantage in strategic ways.
Maybe a girl with a similar power. For example, a girl who can communicate with animals telepathically. They would have a deep understanding of each other because of their shared connection to the animal world. They could go on adventures together, using their powers to help animals in trouble. Their relationship would be based on mutual respect for their powers and a shared passion for animals.
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.
A powerful sorcerer could be the main antagonist. Maybe the sorcerer is trying to gather all the meta - human powers for some dark ritual. He targets Beast Boy first because his power of transformation is unique and could be a key component in the ritual. So, he casts a spell that takes away Beast Boy's powers.
Storm's power over the weather could be enhanced by Beast Boy's animal forms. For example, if Beast Boy turns into an eagle and flies into a storm cloud that Storm has created, he could disrupt the electrical charges in a way that Storm can then manipulate to create a more powerful lightning strike.
Storm's weather - controlling powers and Beast Boy's shape - shifting could interact in a combat scenario. For example, Beast Boy could shift into a large bird and fly into a storm cloud created by Storm to gain an aerial advantage against an enemy.
Maybe in the fanfic, there's a powerful villain who has a device that can nullify superpowers. Beast Boy gets caught in the range of this device and loses his powers. He struggles with feelings of uselessness at first. But then he realizes that his skills as a strategist and his knowledge of the Titans' operations are still valuable, and he uses those to help the team in a different way while also searching for a power restoration method.
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.