Another potential antagonist could be a foreign power that hears about the baby's unique abilities. They might send spies and assassins to either capture the baby or kill it to prevent Ravka from having an advantage. This foreign power could be jealous of Ravka's Grisha abilities and see the baby as a way to tip the scales in their favor if they can control it.
The Apparat could be a main antagonist. He might see the baby as a tool to gain more power and control in Ravka. He could try to manipulate Alina into giving up the baby so he can use it for his own political and religious schemes.
Mal could be a great addition. His relationship with Alina adds an interesting dynamic. He could be jealous of the Darkling's closeness to Alina and cause some tension. Also, Genya. Her skills as a Tailor and her own experiences in the Ravkan court can bring in more political intrigue and a different perspective on the relationship between Alina and the Darkling.
I would recommend 'Shadows of the Heart'. In this fanfiction, the story takes a different turn from the original. It explores what could have happened if Alina had made different choices regarding the Darkling. It's full of interesting plot twists and great character development.
In this fanfiction, the baby could have a special connection to the Sun Summoner power that Alina has. As it grows, it starts to show signs of being able to manipulate light in new and unexpected ways. This draws the attention of various groups, both good and bad. Alina then has to protect the baby while also trying to figure out how to harness its powers for the greater good. She might face challenges from those who want to use the baby's powers for their own evil purposes, and she has to rely on her friends and her own strength to keep the baby safe.
You can find 'Darkling and Alina Fanfiction' on websites like Archive of Our Own (AO3). It has a vast collection of fanfictions for various fandoms, including this one. Just search for the relevant tags and you'll be presented with numerous options.
One common theme is the power struggle. Alina has her own power, and the Darkling has his, and their relationship is often shaped by who has more power at different times. Another theme is redemption. Some fanfics explore whether the Darkling can be redeemed in Alina's eyes or in a more general sense.
You can find 'darkling x alina fanfiction' on websites like Archive of Our Own (AO3). It has a large collection of fan - made stories. Fanfiction.net is also a good place to look. There are often many different takes on their relationship there.
The Foot Clan could be a main antagonist. They might see the baby sister as a way to get to the turtles, either by kidnapping her or using her to blackmail them. Another could be Shredder himself if he returns. He would surely not pass up the opportunity to use the baby sister against the turtles. And Baxter Stockman could also be an antagonist. He might be interested in using the baby sister for some of his mad - scientist experiments.
There could be a rogue Saiyan who has different ideals from Vegeta. This Saiyan might think that the baby should be raised in a more traditional, warrior - like way and try to take the baby from Bulma and Vegeta. Or, an evil scientist who wants to experiment on the baby's unique Saiyan - human DNA could be the main antagonist, creating all kinds of challenges for Vegeta and Bulma to protect their child.
It could be some rogue shinigamis who oppose the idea of rukia and ichigo having a child together. They might see the baby as a threat to the existing power structure in the Soul Society.
In the 'Harry and Daphne baby fanfiction', it could be a new dark wizard who rises to power. This wizard might believe that the baby has some sort of power that could stop him from achieving his goals. So, he makes it his mission to eliminate the baby. Also, there could be some magical creatures that are being controlled by a dark force and are sent to attack Harry, Daphne and their baby because they sense the baby's potential power.