Often, Aizen can still be a major antagonist even in time travel fanfictions. His schemes might extend across time, and Ichigo and Orihime have to figure out how to stop his plans that are messing with different timelines. Another could be the more powerful versions of Hollows from different time periods. They might be stronger and more menacing than the ones Ichigo and Orihime are used to in their original timeline.
The antagonists could also include time - based entities. For example, there could be a being that feeds on the chaos caused by disrupting the timeline. Ichigo and Orihime's time travels create this chaos, and this entity tries to stop them from fixing the timeline because it wants to continue to thrive on the disorder. Additionally, if they travel to a future where an evil version of Ichigo has taken over, that version of himself could also be an antagonist as they try to set things right.
In some cases, an older or more powerful Yhwach could be the main antagonist. If Ichigo and Orihime travel to a time when the Quincy were more dominant, Yhwach might see them as threats to his grand plans. He could use his powers to try and manipulate the timeline to his advantage, and Ichigo and Orihime would have to stop him. There could also be rogue Shinigami from the past who oppose Ichigo and Orihime's attempts to fix the timeline as it goes against their own self - interests.
Some fanfictions might introduce new characters as antagonists. These could be rogue Quincy who believe that Ichigo and Orihime's relationship is a threat to their plans for the spiritual world. They try to separate them to gain an upper hand in their schemes.
In some fanfictions, a powerful Hollow could be the main antagonist. It might target Orihime because of her special powers and Ichigo has to protect her while also dealing with his new - found feelings for her. The Hollow could be constantly attacking them, trying to separate them.
In some fanfictions, they might time travel to an era where the Quincy were more powerful. Ichigo, being part Quincy, might have to face some difficult decisions regarding his heritage. Orihime would be there to support him, and together they could end up changing the course of history between the Shinigami and the Quincy. This could lead to a completely different world in the present, with new alliances and enmities.
The Soul Reapers could be antagonists if they view Ichigo's vampirism as a threat to the balance between the living and the dead. They might try to hunt him down to eliminate this new, unregulated force.
A powerful rogue Soul Reaper could be an antagonist. This Soul Reaper might be jealous of Ichigo's new - found time - control ability and try to steal it for his own dark purposes. Maybe he wants to use it to rewrite history in his favor, eliminating all those who opposed him in the past. Another potential antagonist could be an ancient and evil entity that has been trapped in a time - locked dimension. Ichigo's use of time - control accidentally releases this entity, and it now wants to use Ichigo's power to break free completely and wreak havoc on the world. There could also be a group of Quincy who see Ichigo's power as a threat to their own plans of taking over the spiritual world. They try to stop him from using his time - control ability by any means necessary.
In Ichigo x rukia x orihime fanfiction, Ichigo is typically a complex character with a lot of depth. His relationships with Rukia and Orihime are multi - faceted. Rukia might be shown as a strong - willed and capable partner for Ichigo in the fight against evil. Orihime is usually the one who provides a more tender aspect to Ichigo's life. Fanfiction writers may use different scenarios, like battles or moments of self - discovery, to explore how Ichigo interacts with both Rukia and Orihime, and how their relationships develop over time.
I'm not sure about a specific 'Ichigo and Orihime lemon fanfiction' title off - hand. But you can often find great ones on fanfiction websites. Just search for 'Ichigo Orihime relationship fanfiction' and look through the results. You might have to sift through a bit to find the ones that are well - written and suit your taste.
Some new and upcoming authors can also be great for Hollow Ichigo and Orihime fanfiction. They might bring fresh and innovative ideas to the table. They could take risks with the plot, like creating a completely new backstory for how Hollow Ichigo and Orihime first meet that is different from the original Bleach storyline. These authors can often be found in the 'newly updated' or 'recently published' sections of fanfiction websites.
Often, it could be dark wizards from the past who oppose Harry and Amelia's attempts to change things for the better. For instance, a dark wizard who wants to keep a certain evil spell in place that Harry and Amelia are trying to stop.
It might also be time itself. There could be some sort of time - related magic or force that doesn't want the past to be changed. So Harry and Tonks would have to face this 'antagonist' as they try to rewrite history. This could be in the form of time - locked spells or magical barriers that prevent them from interfering in certain events.