The Soul Reapers in the Soul Society would see Ichigo as a major threat. They would likely mobilize their forces to either capture or eliminate him. Captains like Byakuya would be disappointed as he had come to respect Ichigo to some extent, and now would see him as a traitor.
Rukia would be extremely shocked and disappointed. She has always seen Ichigo as a force for good, and seeing him side with Aizen, who is the main antagonist, would be a huge blow to her. She might try to talk Ichigo out of it at first, and if that fails, she could become his enemy.
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 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.
Yes, there are likely some fanfics with this concept. Fanfic writers often come up with unique and sometimes out - of - the - box ideas. However, this goes against the original storyline of Bleach where there is no such relation between them in the official canon.
One possible plot could be that Ichigo is somehow tricked by Aizen. Maybe Aizen convinces Ichigo that by joining him, he can gain greater power to protect his friends in a different way. Ichigo, being naive in some aspects, might fall for this and start following Aizen's lead. This would lead to a huge shift in the power dynamics in the Bleach universe as Ichigo is a very powerful character.
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.
Well, in 'ichigo and fem aizen lemon fanfiction', Ichigo is often the more emotional and impulsive one. His reactions to Fem Aizen's actions and presence are key elements. He might be drawn to her in a way that challenges his sense of right and wrong. Fem Aizen could be depicted as having a seductive allure that she uses to her advantage. She might also have a more complex backstory related to her femininity in this fan - created world. Her relationship with Ichigo is usually full of tension and a sort of forbidden attraction due to their different natures and the history between Aizen and Ichigo in the original Bleach storyline.
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.
Rukia would probably be both amazed and a bit worried. Amazed at the new power Ichigo has, but worried about how it might change him. Chad would see it as a natural progression of Ichigo's power and be supportive. Orihime might be a bit scared at first, but then she would see the good in Ichigo's new form and try to encourage him.