What could be the possible plot in a fanfic where Ichigo betrays and joins Aizen?

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.

Aizen joining the Shinigami in a fanfic could lead to a complete change in the power dynamics. Since he is a very powerful character, his addition might disrupt the existing balance. He could start training some of the younger Shinigami in his own way, which might be a blend of his devious methods and the traditional Shinigami training. This could create a new generation of Shinigami with a different perspective. Also, his past actions and the knowledge he has gained from his time as an antagonist could be used in unexpected ways. For instance, he might use his understanding of the Hollow world to create a new defense mechanism for the Soul Society against Hollows, but with a hidden agenda that only unfolds later in the story.

A plot twist could be that the place they are captured in is not what it seems. It could be a magical or illusionary prison created by an unknown force. This would make their escape much more difficult as they have to figure out the true nature of the prison. Also, it could turn out that their capture was a self - fulfilling prophecy. Maybe Aizen's actions in the past led to a chain of events that resulted in their capture, but in a way he didn't expect. And Irukia could be the key to their escape, but he doesn't realize it until later in the fanfic.

In an 'Aizen hypnotizes Ichigo' fanfic, the plot could start with Aizen having some grand scheme that requires Ichigo's cooperation, which he obtains through hypnosis. Ichigo might be made to believe that he is on Aizen's side and is working towards a 'greater good'. Meanwhile, characters like Rukia or Urahara could suspect something is wrong with Ichigo. As the story progresses, there could be a series of battles and mind - games as the good guys try to rescue Ichigo from Aizen's control. There could also be some exploration of the nature of hypnosis in the Bleach universe and how it affects Ichigo's psyche.

Maybe Aizen has created a false asylum as part of a new experiment. He lures Ichigo and Rukia into it, and they have to find a way to escape while also dealing with the illusions and traps Aizen has set up. In the process, they could learn more about Aizen's motives and past, and perhaps even find a way to defeat him once and for all.

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.

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.

One possible plot twist could be that Aizen has a hidden motive other than just causing chaos. Maybe he kidnaps Ichigo to use his unique powers for some sort of experiment. For example, he might want to extract the power of Ichigo's Hollow side to enhance his own strength or create a new type of Arrancar. Another twist could be that Aizen is being controlled by an even greater force, and Ichigo's kidnapping is part of a larger plan to draw out other powerful characters in the Soul Society.

Well, in a fanfic where these characters are captured, there could be a lot of intense situations. Maybe Aizen, being the crafty character he is, would try to manipulate the situation to his advantage. Ichigo, on the other hand, would probably be looking for a way to break free and fight back. And Irukia might be using his cool - headedness to assess the situation and come up with a plan. They could also be forced to work together against a common enemy in the fanfic, which would be an interesting dynamic considering their different personalities and powers.

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.