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.
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.
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.
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.
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.
It's hard to say for sure. Bleach Fullbringer is typically aimed at a wide audience, so it's unlikely to have blatant inappropriate stuff. However, it might have some intense or mature themes.
There's a fanfic called 'The Bond of Hearts'. It's set in an alternate universe where Ichigo and Orihime are childhood friends. It shows their relationship evolving over the years. It delves deep into their emotions and how they gradually realize their love for each other. The character development in this fanfic is quite engaging.
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.
One popular plot idea could be an alternate universe where Ichigo and Rukia grow up together as normal humans and gradually discover their spiritual powers later in life. Another could be a time - travel plot where they have to go back in time to prevent a major disaster in the Soul Society. And a third could be a story where they are both sent on a secret mission to the human world together and have to pose as a married couple to blend in.
A possible plot development could be Ichigo being made to fight against his former allies. Under the brainwashing, he might believe that his friends are actually his enemies. Another development could be Xcution trying to extract Ichigo's power while he's in this brainwashed state.
Ichigo would probably be devastated. His trust in relationships would be severely shaken. He might become more withdrawn and focus even more on his battles and training, using it as an escape from the pain of the betrayal. He could also become more cynical about love and people in general.