Raven would probably be the first to sense that something was wrong with Beast Boy. She'd be quiet about it at first, but she'd start keeping a closer eye on him. Maybe she'd make him some herbal tea that she believes has healing properties.
It could be a magically - induced illness. If there are magical forces at play in the fanfiction world, an evil sorcerer or a cursed object might be the culprit. For example, he might have unknowingly touched a cursed amulet that started draining his life force, leading to him getting sick.
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.
There could be. Fanfiction is a vast world, and fans often come up with unique and sometimes unexpected storylines. Beast Boy and Starfire are both popular characters, so it's not unlikely that someone has written a fanfiction with this plot. But it might be hard to find among the countless other fanfictions.
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.
In the fanfiction, perhaps Beast Boy's girlfriend could be a new superhero in town. They meet during a battle against a common enemy. She first sees his brave side when he's in the form of a fierce beast, and she's drawn to him. As they get to know each other more, they start to date. Their relationship could be filled with adventures as they fight crime together and also deal with the normal relationship stuff like jealousy and misunderstandings.
One possible plot development could be that his friends rally around him. For example, Sakura might use her medical skills to try and figure out what's wrong. Another could be that Naruto's illness is somehow related to a new enemy or an old foe's hidden jutsu. It might also lead to Naruto having to rely on others more than he's used to, which could be a big character - growth moment for him.
One possible theme could be responsibility. Beast Boy would have to face up to the fact that he is going to be a father, which is a huge responsibility. Another theme might be acceptance within the Teen Titans team. They would need to accept this new situation and support their friends.
In the fanfiction, Beast Boy could have eaten all of Raven's favorite snacks without asking. This simple act of thoughtlessness might set her off. Or he could have interrupted her meditation multiple times, which is very important to her. Also, if he was being too over - confident in a situation where Raven knew they needed a more careful approach, his cockiness could be a reason for her anger.
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.