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.
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.
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.
Perhaps the time travel is not what it seems. It could be a trap set by an unknown enemy. When Ichigo and Unohana think they are traveling through time to fix something, they are actually being led into a situation where they will be separated and used against each other. This would create a lot of drama and conflict in the fanfiction.
A possible plot twist could be that one member of Team Hebi, say Karin, is actually from the future and has been manipulating the time - travel events all along to achieve a certain goal, like saving her own clan. This would completely change the team's dynamic and their perception of her.
One possible plot twist could be that when they time travel, they accidentally change a key event in the past that has a domino effect on their present. For example, they might prevent the formation of a guild that was crucial in their original timeline. Another could be that they find out they are related in some unexpected way through their time - traveling adventures. Maybe Ultear discovers she has a connection to Natsu's family that was lost in history. Or perhaps they time travel to a future where everything they know is in ruins, and they have to figure out how to prevent it in their present.
One possible plot twist could be that when they travel back in time, they accidentally change a small event that has huge consequences in the future. For example, they might prevent a key meeting between two clans, which then alters the political landscape of the ninja world. Another could be that they find out that one of them was actually from that time originally and had their memories wiped in the future. This would completely change their understanding of themselves and their place in the ninja world.
The story could then develop as Ichigo tries to find a way back home. Along the way, he gets caught up in the politics and battles of the DxD world. He might meet Issei and initially be at odds with him because of their different outlooks on power and combat. But as they face common enemies, they start to work together. Ichigo could also have run - ins with powerful devils like Sirzechs Lucifer, who might be intrigued by his power and want to either recruit him or eliminate him as a potential threat.
Maybe Ichigo is secretly a girl because she's on a secret mission. In the fanfiction, she could be a spy or an undercover agent. Her guise as a boy has allowed her to infiltrate certain groups or places. When her true identity as a girl is discovered, it throws the whole mission into chaos, and she has to find a way to salvage it while dealing with the personal relationships that are affected.
A really cool plot twist would be if during their time travel, they find out that time itself is a sentient being or is being controlled by an unknown force. This force has been manipulating events in the Soul Society and the human world all along. Ichigo and Urahara then have to figure out how to deal with this new discovery, whether to fight it or try to negotiate, while also trying to find their way back to their own time.
A possible plot twist could be that the time travel was all a simulation created by an enemy to deceive the main character. So, the character thinks they are changing the past or future but are actually being manipulated within a false reality.