Perhaps he simply lost his creative inspiration. Fanfiction writing often requires a constant flow of new ideas, and if he's been through a lot of stress or has had some personal issues, like his complex relationship with Lily still haunting him, it could dry up his well of creativity.
It could also be due to some external factors. Maybe there are threats from dark forces that he doesn't want to involve Lily in. If he is getting deeper into the dark arts or some dangerous situations, he might choose to give up on her to protect her from the potential harm that could come her way because of his associations.
The story might take a more somber turn. Snape could become more withdrawn and focus solely on his own pursuits. For instance, he may throw himself into his potions research even more intensely.
One possible plot could be that Snape discovers he has a unique magical ability that allows him to carry a child. He might keep it a secret from everyone in Hogwarts due to fear of ridicule. As he nears the time of giving birth, he would have to find a secluded place within the castle or in the Forbidden Forest. He would endure the pain alone, using his own potions and magic to ease the process as much as possible.
Common themes include the transformation of Snape's character. He might go from a brooding, often unapproachable figure to someone more empathetic and understanding due to the experience of giving birth. Another theme is the exploration of the magical aspect of pregnancy in the 'Harry Potter' world. How does magic play a role in Snape's pregnancy? Is it different from a normal wizard or witch's pregnancy? And also, there could be a theme of family. Snape creating a new family unit and how that family interacts with the existing wizarding families in the story can be an important part of such fanfictions.
It could be a rare potion ingredient. Hermione is known for her academic prowess and might find a special ingredient that Snape has been seeking for his potions research.
Well, a plotline might involve Snape initially being in denial about the pregnancy. As it progresses, he could start to form an unexpected bond with the unborn child. There could also be a plot where other characters find out and either support him or try to use it against him. Maybe Voldemort discovers it and tries to manipulate Snape in a new way because of this situation.
Perhaps Snape gives them the talk about the importance of unity in the face of the dark forces that are always lurking. He could be using his own experiences to make them understand that they should put aside their differences for the greater good. For example, he might talk about how during the war, internal disputes among wizards only made it easier for Voldemort to gain power. By uniting, they could be a stronger force against any potential threats. Also, Snape might use this talk to make them realize that their personal grudges are petty compared to the bigger issues at hand.
Snape would probably use his knowledge of potions. He might have brewed a special pain - relieving potion in advance. He would also use his Occlumency skills to try and block out some of the pain mentally. Since he is a strong - willed character, he would grit his teeth and endure it as best he could while focusing on the task at hand, which is giving birth safely.