Ichigo's character may develop by becoming more confident in his powers. In the past, he has a chance to redo things and learn from his mistakes. He could become more strategic in battles, using his transcendent abilities more effectively.
Well, in such a fanfiction, a key plot point could be Ichigo's discovery of his new transcendent powers in the past. Maybe he has to face old enemies with his enhanced abilities. Another point could be how his presence in the past alters the future he came from. For example, his interactions with past characters might change their fates, which in turn could have a butterfly effect on the overall Bleach universe.
In RWBY fanfiction, a transcendent Ichigo can bring a whole new dynamic. He could potentially have powers that far exceed the normal capabilities in the RWBY universe. His sword skills, for example, might be on a level that no one in RWBY has ever seen. This could lead to interesting interactions with the RWBY characters, like Ruby being in awe of his combat abilities.
Ichigo's character could develop a greater sense of patience. In different time periods, he might have to wait for the right moment to act or to understand the events around him. For example, if he travels to a more primitive time in the Soul Society, things may not happen as quickly as he's used to in his own time. This would force him to be more patient. He could also become more empathetic as he experiences the hardships of different eras and the people in them.
One common theme is power - up. Since Ichigo is becoming transcendent, it often involves him gaining new and extraordinary powers that set him apart from his normal self in the original series. For example, he might have the ability to manipulate time or space.
One possible plot twist could be that Ichigo travels back in time and accidentally changes something that makes him not become a Soul Reaper in the first place. This would completely change his destiny and the future of the Soul Society. Another could be that he travels to a time when an ancient evil was first sealed away, and his presence there somehow weakens the seal, leading to a new threat emerging in his present time.
He fits by being an overpowered character that disrupts the normal power balance. This can make for interesting stories as the RWBY characters have to adapt.
One great 'Bleach Ichigo Transcendent Crossover Fanfiction' is 'Ichigo in the World of Naruto'. It shows Ichigo's powers in a completely different ninja - filled world. His reiatsu stands out among the chakra - using ninjas. It's an exciting read as he has to adapt to new rules and also face off against powerful foes in the Naruto universe.
One interesting character to crossover with Ichigo is Light Yagami from Death Note. The clash between Ichigo, who is a protector, and Light, who is a morally ambiguous character with the power to kill, would be very engaging. Saitama from One - Punch Man could also be a great choice. Saitama's overpowered nature and Ichigo's journey to become stronger would make for an interesting story. And what about Alucard from Hellsing? His vampire powers and Ichigo's powers related to the afterlife would create a really dark and exciting crossover.
Aizen Sousuke would be an interesting crossover character. Since he's so powerful and manipulative in the Bleach universe, seeing him in a different setting with Ichigo could create some really complex and intense storylines. They could be forced to work together against an even greater evil in a crossover universe.