Well, one possible plot twist could be that Tsukishima's brainwashing of Ichigo is part of a larger plan by a hidden villain. Maybe this villain wants to use Ichigo's power for some evil purpose. Tsukishima might be forced to do this brainwashing against his will in some way.
Another interesting plot twist could be that the brainwashing is not just about controlling Ichigo's actions, but also about changing his perception of the world. So, Ichigo might start seeing his friends as enemies and his enemies as friends. As the story progresses, he could gradually realize the truth and have to make some really tough decisions about who to trust.
Perhaps the fullbringers are being controlled by a new, unseen enemy. This enemy wants Ichigo out of the picture, so they use the fullbringers to brainwash him. Ichigo, while brainwashed, starts to cause destruction in the Soul Society. His actions draw the attention of the Soul Reapers, who are initially unaware that he's been brainwashed. As the story progresses, some of the more perceptive Soul Reapers figure out what's going on. They then team up with Ichigo's friends from Karakura Town to find a solution. This could involve a lot of exploration of Ichigo's mind and his past relationships to find the key to breaking the brainwashing.
Rukia would probably be very suspicious right away. She knows Ichigo well and would notice the strange changes in his behavior. She might try to confront him and figure out what's wrong.
One possible reason could be that Tsukishima is under some sort of mind control or influence from a more powerful and malevolent force. Maybe he has been deceived into believing that Ichigo is a threat to something he holds dear.
Ichigo might initially be in shock. He could be unable to believe that Tsukishima, who might have been a friend or at least an acquaintance, has turned on him.
One possible fanfic could be where the fullbringers use some sort of spiritual power to brainwash Ichigo. Maybe they want to turn him against his friends in Karakura Town. They could manipulate his memories of his battles and his relationships, making him believe that his friends are actually his enemies. This would lead to some really intense fight scenes as his friends try to break the brainwashing and bring Ichigo back to his senses.
One possible plot twist could be that Ichigo, now as a girl, has to deal with different power dynamics. Maybe her reiatsu (spiritual pressure) is affected in a new way. For example, it could become more refined or have a different color, which would attract different kinds of Hollows or Soul Reapers. Another twist could be in her relationships. Her friends might initially be shocked and not know how to interact with her. Uryu, for instance, might be more hesitant to be around her at first. Also, the Soul Society's reaction could be interesting. They might be suspicious of this change and think it's some kind of new threat or a trick by the Quincy. Overall, the story could explore how Ichigo's personality remains the same despite the physical change, but she has to adapt to the new social and power - related situations.
Well, sometimes in these fanfics, Ichigo is betrayed by his Hollow side. It could be that the Hollow part of him has its own agenda and decides to use Ichigo's friends as bait to draw out more powerful opponents. This would make Ichigo feel extremely guilty as he's the one who brought this danger to his friends. Also, there could be a plot twist where Ichigo is betrayed by a new character introduced in the fanfic. This new character might have pretended to be his ally all along but was actually working for a powerful antagonist. This would make Ichigo realize that he can't trust people so easily.
Well, it could be about their adventures together after the main events in Bleach. Maybe they go on a mission to a new realm and face new enemies while strengthening their bond as friends and comrades.
Well, a really interesting plot twist could be that Yoruichi is actually a reincarnation of an ancient soul that was once bound to Ichigo's family in a very different way. And their marriage is a way to restore some sort of balance in the world of Bleach. Also, what if Ichigo suddenly starts to lose his powers during the wedding preparations? It could be due to some sort of curse or a side - effect of their impending union. Then they have to figure out how to get his powers back without cancelling the wedding.
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.