In 'The Parallel Worlds' fanfiction, the plot twist is that Ichigo and Rias are actually from parallel universes that are starting to merge. They have to figure out how to stop the merge while also dealing with their feelings for each other. This creates a very unique and complex situation for them to navigate.
One possible plot idea could be that Ichigo and Rias team up to fight a common enemy that threatens both their worlds. Maybe a powerful dark force that neither of their usual allies can handle alone.
A plot twist might involve Rias having to choose between her duties in the Gremory clan and her love for Naruto. This could create a lot of internal conflict for her and also put a strain on their relationship. For example, she might be forced to enter an arranged marriage for the sake of her clan, but she really loves Naruto. This would lead to a complex and emotional storyline.
One interesting plot twist could be that Ichigo's zanpakuto spirit interacts in a unique way with Naruto's chakra. Maybe it gives Naruto the ability to use some Hollow - like powers, which would completely change the dynamic of his battles. Another could be that Ichigo's past memories start to surface in Naruto's mind, leading to confusion and also new insights into combat and relationships. A third one might be that the concept of the Soul Society in the Bleach world somehow merges with the ninja world in Naruto, creating a whole new set of rules and power hierarchies.
Perhaps Byakuya loses his powers and Ichigo has to help him regain them. This role - reversal would be very interesting as Ichigo becomes the teacher and Byakuya the student. They would have to work together in a whole new way, and Ichigo would have to prove himself to Byakuya in a different context.
In some fanfics, a great plot twist is when Ichigo's Hollow powers are misinterpreted by the DxD characters as some sort of devilish power. This leads to a lot of misunderstandings and conflict at first. Another interesting plot twist can be when Ichigo somehow ends up being related to an important figure in the DxD world through some convoluted backstory.
Perhaps a plot twist where Dark Rukia discovers that Hollow Ichigo has a connection to her past that she was unaware of. Maybe he holds the key to unlocking a part of her memory that was sealed away, which changes their dynamic completely as they try to figure out what it all means.
In 'Twisted Fates', the plot twist is that the powers of Ichigo and a certain devil in High School DxD get merged accidentally. This leads to a new form of power that no one expected. It causes a huge shift in the power balance and makes both the allies and enemies of the characters scramble to adjust to this new situation. This fanfiction then delves into how they all cope with this unexpected development.
One interesting plot twist could be that when Ichigo is revived, he comes back with new and unexpected powers. For example, he might have the ability to control time on a small scale, which he uses to outwit his enemies in battle. Another plot twist could be that his revival is part of a larger, more sinister plan by an unknown force. This force might be using Ichigo as a pawn to achieve their own goals, and Ichigo has to figure out what's really going on while dealing with his new life. A third plot twist could be that Ichigo's revival brings back memories from a past life that he didn't know he had, and these memories hold the key to defeating a powerful enemy that has emerged while he was 'dead'.
One common plot twist is when the powers of Naruto and Ichigo somehow merge during the battle, creating a whole new set of abilities that neither of them expected. This can change the entire dynamic of the fight.
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.