Who could be the main villain in a Zoro and Robin baby fanfiction?

In this fanfiction, the pregnancy could be a result of a deep emotional bond that developed between Zoro and Robin over time. The crew might be initially shocked but then very supportive. The baby could become a symbol of a new beginning for the Straw Hats. There could be challenges like protecting the baby from other pirates or the World Government who might see the child as a threat due to its parents' reputations. Also, the baby could bring out a softer side of Zoro and Robin, and they could learn a lot about themselves as they become parents.

A plot for this fanfiction could be that Robin and Zoro keep the pregnancy a secret from the crew at first. But as time goes on, it becomes harder to hide. Meanwhile, they are also facing a new island with strange customs and threats that are related to Robin's pregnancy in some way. For example, the island has a law that pregnant women must perform a certain ritual, and if they don't, something bad will happen to the baby.

The Brotherhood of Mutants could also pose a threat. They might see the baby as a symbol of the X - Men's future and want to eliminate it. Or they could try to turn the baby to their side, using its powers for their own radical mutant - rights agenda.

Perhaps a stray dog. The dog is always looking for something to chase or bother. When it sees the mommy cat taking care of the baby robin, it decides to make trouble. It tries to scare the cat away from the robin so that it can have fun chasing the small, helpless bird.

In the fanfiction, the baby could have some unique abilities inherited from both Zoro and Robin. For example, it could have Zoro's sense of direction in combat and Robin's ability to read ancient texts. This could lead to many interesting adventures as the crew tries to understand and help the baby develop these powers.

It could be a future version of an existing villain who has somehow gained even more power. For example, a more advanced and menacing version of the Light. They might be the ones causing the future problems that Future Robin is trying to prevent, and their actions in the present are just the start of their grand plan.

Ink could be a great main villain. He's a character from the Batman Beyond series known for his ability to manipulate ink - like substances. He could target Robin specifically, seeing him as a weak link in the Batman - Robin duo. His powers would pose a unique challenge for Robin to overcome.

It could be Trigon. Since Raven has a connection to him, he might see the baby as a way to get revenge on Raven or as a powerful tool for his evil plans. He could try to corrupt the baby's powers for his own use.

It could be a rogue ninja who is jealous of Naruto and Hinata's success and wants to use their baby girl to get revenge. Maybe this villain has a grudge against the Uzumaki clan and sees the baby girl as a way to strike at them.

It could be an envious time traveler who wants to steal the baby for some unknown but probably malicious reasons. Maybe they think the baby has some special power or is a key to something valuable in the time - space continuum.