Aizen might be initially dismissive, thinking it's just another mortal event. But as he realizes the potential power the baby could possess, he could become envious or see it as a new pawn in his schemes. He might start observing from the shadows, waiting for the right moment to make his move.
The plot might involve Aizen being released from his imprisonment somehow and seeing the new - born as a threat. Since Ichigo and Rukia are powerful in their own right, their child could potentially grow up to be even more powerful. Aizen could try to manipulate the situation to either turn the baby to his side or eliminate it. But Ichigo and Rukia's love for their child would be a strong force against Aizen's evil intentions.
Their friends would probably be really surprised at first. Orihime might be super excited and start making all kinds of baby clothes and accessories for them. Chad would be happy for them in his quiet way, and Ishida might be a bit more reserved in his reaction but still offer his support.
At the start, Ichigo would be defiant. He would not show any fear in front of Aizen. He might taunt Aizen, trying to get him to reveal his true intentions. As time goes on, he would start to feel a sense of isolation, but this would only strengthen his resolve. He would use his inner Hollow powers more freely, as he has no other option. He would also be constantly thinking about ways to turn the tables on Aizen and escape, and would be ready to take on any challenges that come his way during his captivity.
Ichigo might become more reliant on Rukia's knowledge of the Soul Society when it comes to teaching their daughter about her heritage. Rukia, in turn, would rely on Ichigo's human - side experiences to give their daughter a well - rounded life. Their relationship would evolve into a partnership in parenting, with mutual respect growing even more.
Ichigo would likely be defiant. He's a fighter and would not easily submit to Aizen. He'd probably be constantly looking for a way to escape and would be sizing up Aizen's every move, trying to find a weakness.
The new addition to their family could also make them more open about their feelings for each other. They might realize how much they truly care for one another while watching their baby grow. This could lead to more affectionate moments between them and a more solid romantic relationship.
Many of the captains would be on the edge of their seats. They'd be closely observing every move Ichigo makes. Ukitake might be silently cheering Ichigo on, hoping that he can succeed where they might have failed in the past against Aizen. And Hitsugaya would probably be analyzing Ichigo's techniques to see how he can improve his own skills.
Orihime would probably be over - the - moon excited. She's always been a kind - hearted and emotional person. She would immediately start knitting baby clothes or making cute little charms for the baby. Chad would be very supportive as well. He would offer his strength to protect Rukia during her pregnancy. Uryu might be a bit more reserved at first, but deep down he would be happy for them. He might use his Quincy skills to create some sort of protective item for the baby.
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.
Perhaps the baby would have a unique appearance, with a blend of Ichigo's orange hair and Rukia's dark hair. In the fanfiction, the baby could be a bridge between the human world and the Soul Society. It might develop new abilities that no one has seen before, leading to a whole new set of adventures for Ichigo and Rukia as they help their child navigate this newfound power and its place in the two worlds.