The Hive Five could also be the main villains. They might be jealous of Beast Boy's growth and potential. They could team up and try to take him down. Each member of the Hive Five has their own unique abilities that could pose a challenge to Beast Boy. For example, Jinx could use her bad luck powers to disrupt Beast Boy's transformations, while Gizmo could create technological traps to catch him.
Slade could be a great main villain. His combat skills and strategic mind would pose a huge challenge for Beast Boy. Beast Boy would have to constantly outwit and outmaneuver Slade, which would really bring out his badass side as he tries to protect his friends and the city from Slade's evil plans.
A 'Beast Boy Full Potential Fanfic' could take him on a journey of self - discovery. He could travel to different dimensions or places on Earth that are full of unique animals. By interacting with these animals, he learns to harness their essence and add it to his transformation abilities. He could also meet other shape - shifters and learn from them. This would lead to him evolving his powers to a level where he can even combine multiple animal traits in one form, like having the speed of a cheetah and the strength of a gorilla at the same time.
It could be a mad scientist who wants to study Beast Boy in his beast form and keep him caged for experiments.
A possible ally could be a character with healing powers. Since Beast Boy often gets into dangerous situations while using his shapeshifting, someone who can heal his injuries would be very useful. For example, a healer who can also communicate with animals on a spiritual level, similar to Beast Boy's connection to them.
It could be Trigon. Since Raven has a connection to him, he might see the baby as a way to get revenge on Raven or as a powerful tool for his evil plans. He could try to corrupt the baby's powers for his own use.
The main villain could be a time - traveling version of one of his regular foes, like Slade. Maybe Slade from the future has developed a way to manipulate time and he's going back in time to stop Beast Boy from ever becoming a Titan or to change events in his past that will make him easier to defeat. Another option could be an ancient evil that was sealed away in the past and Beast Boy's time travel accidentally releases it. It then tries to take over the present by changing key events in history. And it could also be a future version of Beast Boy himself who has been corrupted by power and is trying to change the past to make himself even more powerful.
Trigon could also be the main antagonist. Given his immense power and his connection to Raven, he could target Beast Boy as a way to get to Raven or simply to cause chaos among the Teen Titans. His demonic powers could be too much for Beast Boy to handle, ultimately resulting in Beast Boy's death.
The Penguin could be a great main villain. He could be up to his usual schemes in Gotham, like stealing valuable artifacts. When the little boy witnesses one of his crimes, the Penguin tries to silence him. Batgirl then has to protect the boy and stop the Penguin's evil plans.
It could be Slade. Slade might manipulate situations to make Beast Boy look bad in front of his friends. He could spread false rumors or set up situations where Beast Boy fails, which would make Beast Boy angry at being wrongly accused and at the fact that his friends might start to doubt him.
Maybe some mad scientist who wants to experiment on Beast Boy's shape - shifting DNA. He could be looking for a way to transfer the ability to himself or create some sort of mutant army. Another option is that it's a group of aliens who mistake Beast Boy for a dangerous creature and capture him to study. They might not understand that he's a sentient being with friends who will come looking for him.