The Soul Society itself could be the main antagonist. If Ichigo kidnaps Rukia because he thinks they are going to harm her unjustly, then the Soul Society's rules and its enforcers would be the opposing force.
Itachi himself could be seen as the main antagonist in this scenario as he is the one carrying out the kidnapping. He is acting against Sakura's will and causing a dangerous situation for her.
Laxus could be seen as the main antagonist in this scenario since he is the one who kidnaps Lucy. However, it could turn out that there is a greater force behind his actions, in which case that unknown entity would be the true antagonist.
One possible plot could be that Ichigo kidnaps Rukia out of a desperate need to protect her from some unseen and extremely dangerous threat in the Soul Society. He might believe that the Soul Society's own mechanisms for safeguarding her are insufficient.
The antagonist could be Gajeel himself, in a sense. Even though he might have good intentions in kidnapping Levy, his actions are wrong. His own internal struggle between his good intentions and his bad methods could be seen as a form of self - antagonism. For example, he might be constantly battling with the guilt of scaring Levy while also believing that what he's doing is for her own good.
One idea could be that Aizen kidnaps them to use Ichigo's power and Rukia's knowledge of Soul Society for his own dark plans. He might want to study Ichigo's unique Hollow - Shinigami powers up close and use Rukia as a bargaining chip with Soul Society.
Itachi himself could be seen as the main antagonist in this scenario as he is the one performing the kidnapping. His actions are against Sakura's will and are likely to cause distress and conflict.
The main characters are clearly Ichigo and Rukia. Ichigo is a powerful character with his unique abilities, and Rukia is a Soul Reaper. In this fanfiction, they are at the center of the story due to the situation of Rukia being pregnant by Ichigo.
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.
Ichigo and Rukia are at the heart of it. Their family unit might include other Soul Reapers who are like extended family. There could be a story where an old enemy of Ichigo's resurfaces and targets his family. In this case, all the main characters, including Ichigo, Rukia, and their family members, would have to band together to face the threat.
The main characters are Ichigo and Rukia. Ichigo is often a headstrong and brave character with a strong sense of justice. Rukia, on the other hand, is a complex character with her own set of values and a deep connection to the spirit world.