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.
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.
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.
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.
In 'ichigo and aizen lemon fanfiction', the focus is usually on the dynamic between Ichigo and Aizen. It might play with the idea of their rivalry in a more intimate or emotional context. This can add new layers to their characters. However, not all such fanfictions are well - written. Some might over - emphasize the 'lemon' aspect and lose the depth of the characters' relationship.
Rukia could be one. Since she has a close relationship with Ichigo, she might be shown cheering him on or even having some sort of role in the fight. Uryu could also be highlighted. He's a powerful Quincy and might have some unique insights or actions during the fight. Orihime, with her healing and shield - like abilities, could potentially play a role in protecting Ichigo or the others during the battle.
Aizen ended up being confined and his powers were controlled. He was no longer a threat. This conclusion came after a long and intense battle, showing that justice prevailed in the Bleach universe.
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.