Cell could also be an antagonist. He has an interest in powerful Saiyan genetics. The baby, being related to Goku, would be of great interest to him. He might try to absorb the baby to gain even more power, creating a big conflict for the heroes to deal with.
There could be new antagonists created specifically for this fanfiction. For example, an evil scientist who is jealous of Bulma's intelligence and wants to steal her inventions. This new villain could team up with other existing villains in the Dragon Ball universe to create chaos and try to break up Goku and Bulma's marriage.
Wart could also be an antagonist. He might try to take over the Mushroom Kingdom where Baby Mario and Baby Peach live. He could send his minions to stop the two from getting help or finding special items that can defeat him.
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.
Some of the remaining members of the Akatsuki who still hold grudges against Sasuke or see the baby as a threat to their plans. They might target the baby to get revenge on Sasuke or to use it as a bargaining chip. For example, if there are any Akatsuki members who were loyal to Itachi and don't like Sasuke's new life with Naruko, they could cause trouble for the family.
Voldemort's remaining followers could be antagonists. They might see the baby as a new threat or a potential weapon. They could try to kidnap the baby to use its power for their own dark purposes.
The other vampire factions could be antagonists. They might see Klaus and Caroline's baby as a threat to their power or way of life. For instance, if the baby has some extraordinary powers, they could fear that it would disrupt the balance of power among vampires. So, they might try to stop the baby from being born or even try to take control of it once it's born.
One interesting storyline could be about the baby having unique powers inherited from both Goku and Bulma. For example, the baby might have Goku's strength but also Bulma's intelligence, leading to some comical and heroic situations as it grows up.
Definitely include Whis. His knowledge and his ability to train Goku can be a great plot point. Beerus, the God of Destruction, can also be included. His temper and his power can create high - stakes situations in the fanfiction. Additionally, Chi - Chi could be added. Her possessive love for Goku can create conflict when it comes to Bulma's friendship with Goku. And Future Trunks is another character that can bring a sense of mystery and urgency to the story with his knowledge of future events.
One type of antagonist is a mad scientist - type character who wants to experiment on Goku and Fasha to understand their Saiyan powers. This character might create all sorts of dangerous monsters to try and capture them. Another could be a powerful demon from another dimension who is drawn to their power and wants to consume it to become even more powerful.
Frieza or his remnants could be antagonists. They might see the unborn child as a potential threat in the future, especially if it has strong Saiyan powers. So they could plan to attack Bulma to prevent the birth.