Buffy would likely become even more protective and fierce. Her slayer side would blend with her motherly instincts. Willow would grow in confidence as she uses her magic to care for the baby. She might also become more assertive in protecting their family.
The plot might involve Buffy and Willow dealing with the reactions of their friends. Some might be supportive, like Xander, while others, like Giles, could be more cautious. They could also face a threat from an old enemy who sees the baby as a powerful weapon. As the pregnancy progresses, they grow closer together and learn more about themselves and their relationship.
Friendship is a very common theme. Buffy and Willow have a strong bond in the series, and fanfictions often explore the depths of that friendship, how they rely on each other in tough times.
In many Buffy and Angel fanfictions where they have a baby, it often explores new dimensions of their relationship. It could show them dealing with the challenges of parenting while also facing the supernatural threats they are so used to. Maybe the baby has some unique powers that attract both good and bad forces, leading to a whole new set of adventures for Buffy and Angel.
Fanfiction.net is also a good place. There are many Buffy - related fanfics there, and you can use the search function to specifically look for Buffy x Willow ones. Just be aware that the quality may vary.
One great Buffy x Willow fanfiction is 'Soulmates Beyond the Grave'. It delves deep into their relationship, exploring how their bond might develop in a post - apocalyptic setting. The story has a great plot with a lot of character development.
Another interesting one is 'Buffy and Willow: Unbreakable Bond'. It's more of a character - study type of fanfiction. It analyzes their relationship from different angles, like how their past experiences shape their present interactions and how they grow as individuals while being there for each other.
Perhaps the baby could inherit some of Buffy's slayer abilities and Angel's vampire strength. This would make the baby a very powerful and unique being. In the fanfiction, they would have to figure out how to raise and train the baby, while also keeping its existence a secret from the wrong people.
Romance can also be a common theme. Some fanfics develop a romantic relationship between Buffy and Willow, exploring how their dynamic could change in a romantic context. This can involve dealing with the reactions of other characters in the Buffy world.
Friendship is a very common theme. Their deep bond as friends is often explored further, showing how they rely on each other. For example, in many fanfics, they are there for each other during emotional breakdowns.