The H.I.V.E. Five could be the main villains as a group. Beast Boy would be outnumbered but his shapeshifting skills could give him an edge. He'd have to take on each member's unique abilities one by one, and in the process, show his badass nature as he battles against this group of super - villains.
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.
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.
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.
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.
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.
Perhaps a fanfic where Beast Boy has to prove himself to a new group of heroes. He goes through a series of trials that really test his limits and his badass side comes out as he overcomes each one, showing his unique combat skills using his shapeshifting.
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.
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.
Obviously, Beast Boy is likely to be one of the main characters. Since it's in the title, he's probably at the center of the story. He could be the one wearing the mask and dealing with whatever situations it brings.