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.
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.
Well, one possibility is that Aizen somehow convinces Ichigo that his way is the only way to protect his friends. Maybe he shows Ichigo a false future where all his friends die if he doesn't side with Aizen. Ichigo, being desperate to save them, might then join Aizen, thinking he can change things from the inside.
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.
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.
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.
One common theme is the power struggle. Since Aizen is a very powerful character, and Ichigo (even as a female version) has her own strength. Their interactions often involve a push - and - pull regarding power. For example, in many fics, the female Ichigo tries to resist Aizen's control.
You can try searching on popular fanfic platforms like Fanfiction.net or Archive of Our Own. Just use the keywords 'Aizen - Obsessed Ichigo Rukia' in their search bars, and you might find relevant fanfics.