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.
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.
A fanfiction could also focus on their relationships outside of being superheroes. They could be having a normal day at the Tower, just hanging out. Starfire could be trying to cook something new from her home planet, Raven could be reading a book in the corner, and Beast Boy could be cracking jokes, which at first annoys Raven but then she starts to laugh.
Their relationship would likely become more serious. They would have to start thinking about the future in a more concrete way. There would be a new level of responsibility and commitment between them. They might have to cut back on some of their superhero activities to focus on the pregnancy.
One great 'Starfire x Beast Boy fanfic' is 'A Different Kind of Bond'. It shows their relationship developing in a really unique way. The story has a lot of heartwarming moments as they learn more about each other.
A great fanfiction concept could be a time - travel story. They accidentally get sent back in time and have to find a way to get back to their own era. Along the way, they meet younger versions of themselves or other important characters. Beast Boy might be excited to see how things were in the past, Raven could be more cautious, and Starfire would be curious about the different cultures and technologies.
One plot idea could be that they accidentally get pregnant due to some sort of alien or magical mishap. For example, a strange alien device that was meant for something else malfunctioned and caused Starfire to become pregnant. Another idea is that they decide to have a baby as a way to bring their two different cultures (Tamaranian and Earth) closer together. They could face challenges from their friends and family who might not be ready for them to become parents at such a young age.
One scenario could be that the kiss is a spur - of - the - moment thing during a battle. They think they're about to die, so they finally admit their feelings. After that, they have to quickly get back to fighting and deal with the aftermath of the battle while also processing their new relationship. Another scenario is that it's a planned, romantic moment. Maybe Beast Boy sets up a whole date - like scenario on the roof of Titans Tower and then they share a kiss under the stars.
Another storyline could involve her friends' reactions. Her teammates might be overly protective, like Robin being extra vigilant about her safety. They could also be helping her with things she can't do easily during pregnancy, like heavy lifting or flying long distances if it becomes too strenuous.
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.