Well, in a fanfic where Aizen joins the Shinigami, he might actually have a change of heart. Maybe during his time away, he had an epiphany. He could start using his intelligence and powers to actually help the Shinigami. For example, he could help in researching new ways to purify Hollows more efficiently. However, his past sins won't be easily forgotten by all. There would be those who constantly keep an eye on him, waiting for him to slip up. And his relationship with other key Shinigami characters would be very interesting. He might form an unlikely alliance with someone like Urahara, and together they could come up with some revolutionary ideas for the Soul Society, but also with a lot of tension and mistrust underlying their partnership.
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.
Some characters like Ichigo would be extremely suspicious. Given his past battles with Aizen, he would not trust him easily. Ichigo might keep a close eye on Aizen's every move, waiting for any sign of his old devious ways to resurface. On the other hand, some of the more naive Shinigami might be easily fooled by Aizen's charm. They could see his joining as a great opportunity and welcome him with open arms, not realizing the potential danger.
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.
Well, a plot twist could be Batman's technology being hacked by an unknown enemy. Since he is known for his high - tech gadgets and strategies, if his systems were compromised, it would put the whole Justice League in jeopardy. Maybe the hack reveals some of Batman's deepest secrets, which could either cause a rift within the League or bring them closer together as they deal with the fallout. Another idea is that Batman discovers a long - lost relative who has ties to one of the Justice League's main enemies. This would create a moral dilemma for Batman and add a whole new layer of complexity to the story.
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.
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.
One possible plot twist could be that Sakura initially joins Orochimaru to gain more power to protect her friends, but she soon discovers that Orochimaru has ulterior motives regarding her unique chakra abilities. For example, he might want to use her as a vessel for some dark jutsu. Another twist could be that Orochimaru's henchmen start to develop loyalty towards Sakura instead of him because of her strong will and medical skills. This could lead to a split within Orochimaru's group, with some siding with Sakura and others remaining with Orochimaru.
One possible plot twist could be that Draco has to constantly prove his loyalty to the Order. Since he was a former Death Eater, the members might be skeptical at first. For example, they could give him dangerous tasks to test his commitment. Another twist could be his relationship with Harry Potter within the Order. They might start as reluctant allies but gradually become friends, which would be a huge shift from their school - time enmity.
One possible plot twist could be that Naruto has the ability to control the souls of the dead, inherited from his mother, the Fem Shinigami. This could lead to him being able to communicate with past Hokages in a more direct way than just through their chakra imprints. Another plot twist might be that Naruto has to face an enemy who is also related to the Shinigami realm and his connection to it is the key to victory.
One possible plot could be that Naruto discovers a hidden realm where Shinigami exist. He meets a particular Shinigami and is immediately drawn to their unique powers and personality. They start to form a bond as Naruto tries to understand the Shinigami's world and how it relates to his own.
Well, it could be about Naruto having some special powers or abilities related to being the son of a Shinigami. Maybe he can communicate with the dead more easily or has some sort of death - related jutsu.