The First Evil could also be a great antagonist. It could try to corrupt the baby, seeing it as a new vessel to spread chaos. Since the baby would be part of Buffy and Faith, it would be a prime target for the First Evil's schemes.
A new powerful demon could be introduced. This demon might sense the power of the baby and want to either destroy it out of fear or use it to take over the world. The demon could be constantly trying to kidnap the baby, making Buffy and Faith's lives a living hell as they fight to protect their child.
Perhaps the story could be about Buffy and Faith being in a relationship first. Then, they decide they want to have a baby together. But it's not easy in their world. They may have to deal with the Watchers Council's disapproval. They'd need to find a safe place to raise the baby, away from the demons and vampires that are constantly after them. And they'd have to learn to rely on each other more than ever as they navigate this new chapter of their lives.
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.
One possible storyline could be about the moral and emotional turmoil that Faith goes through. She would have to deal with the unexpected pregnancy and the fact that the baby is related to Buffy in some way. Another could be how the Buffy - related characters react to this situation, like Willow trying to help with magic or Giles being his usual wise self in giving advice.
I'm not sure specifically about 'Faith pregnant with Buffy baby' fanfiction. Fanfiction can be quite diverse and spread across many platforms. You could try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net. They have a large collection of Buffy - related fanfics, but it might take some time to find one with this exact plot.
Another vampire family that is jealous of the Mikaelson's power and influence. They see the baby as a threat to their future plans. They might try to kidnap the baby and use it as a bargaining chip to gain more power in the vampire world. They would be constantly plotting against the Mikaelsons to get their hands on the baby.
Danzo could be a great antagonist even when Naruto is a baby. He might see Naruto as a threat to his plans for Konoha's future and try to get rid of him secretly while making it look like an accident.
It could be Hinata. Since they are in a relationship in the original series, it would be a natural choice. Hinata would be a loving and caring mother, and her gentle nature would be a great influence on the baby.
Often, Buffy and Faith kiss fanfiction can be a form of wish - fulfillment for fans who see the potential in their relationship. It can show different interpretations of their personalities. For example, a fanfic might show Faith being more vulnerable through the kiss, or Buffy being more open to a different side of herself. This kind of fanfiction also allows for creativity in exploring different settings and scenarios where this kiss takes place.
It could be a jealous student who wants to undermine Hermione's intelligence. They might have cast the spell to turn her into a baby thinking it would be a great way to get her out of the way. This student could be someone who is always overshadowed by Hermione in class and exams.
A powerful dark wizard could be a great antagonist. He might want to use the Scarlet Baby's unique energy for his dark magic. He could send his minions, like flying monkeys or shadow serpents, to capture the baby.