Yhwach could also be a main antagonist. He has grand plans for the world and his power to absorb and manipulate souls is a huge threat. He would see Ichigo, Byakuya and Stark as obstacles in his way. His army of Quincy could also pose a significant challenge to our trio, and his very existence challenges the balance of power in the Bleach universe.
Maybe a new, original character could be the antagonist. This character could be a powerful hybrid of Soul Reaper, Arrancar and Quincy powers. They could have a vendetta against the Soul Society, Hueco Mundo and Ichigo in particular. Their unique powers would make them a formidable foe for Ichigo, Byakuya and Stark, and their backstory could be something that ties into the secrets and lore of the Bleach world in a new and interesting way.
The plot might also revolve around a mystery. Suppose there's a strange phenomenon occurring that seems to be related to their powers or their pasts. Ichigo, Byakuya and Stark might have to team up to solve this mystery. They could travel to different realms in search of clues. Along the way, they could have some interesting interactions. For example, Byakuya could be initially reluctant to cooperate with Stark because he is an Arrancar, but Ichigo could act as a mediator, trying to get them to put aside their differences for the greater good.
It could be a group of Quincy who have decided that Ichigo and Yoruichi are a threat to their new order. They believe that Ichigo's hybrid nature and Yoruichi's knowledge and power are things that could disrupt their plans to reshape the spiritual world. So, they start a campaign to eliminate them. This group of Quincy is led by a fanatical leader who is willing to do whatever it takes to achieve their goals, even if it means using dark and forbidden Quincy techniques.
There could be a group of corrupted Quincy who view Ichigo's resurrection as an abomination. They believe that his power goes against the natural order of things. So, they band together to try and eliminate him. They might have some new and dangerous weapons or techniques specifically designed to counter Ichigo's resurrected powers. This group could be led by a very powerful Quincy who has a personal vendetta against Ichigo or the Soul Reapers in general.
It could be a villain from Harry's world who is jealous of his new connection with the Starks. Maybe a dark wizard who wants to steal Stark technology to enhance their own dark magic.
Ichigo's family on the other hand, would also be surprised. His father might be a bit more open - minded though, given his own strange abilities and knowledge of the spirit world. His sisters would likely be excited about having Byakuya as a brother - in - law, especially if they see how much Ichigo cares about him. Overall, Ichigo's family would probably welcome Byakuya once they get to know him better.
A dark and mysterious sorcerer could also be the main antagonist. The sorcerer has a long - standing grudge against Byakuya's family or the place he protects. Using dark magic, the sorcerer sets up a series of deadly traps and curses, which ultimately lead to Byakuya's death.
In fanfiction, the relationship between Ichigo and Fem Byakuya can be really interesting. It often explores new dynamics that are different from the original story. Writers might play with their personalities and how they interact in various situations, like during battles or in more personal, emotional moments.
The Soul Society itself could be the main antagonist. If Ichigo kidnaps Rukia because he thinks they are going to harm her unjustly, then the Soul Society's rules and its enforcers would be the opposing force.
Perhaps a vampire scientist who is conducting unethical experiments on both humans and vampires. Ichigo gets in his way when he discovers what the scientist is doing. This antagonist could be using his scientific knowledge to create powerful and dangerous hybrids. He believes that he is creating a new and better vampire - human world order, but his methods are causing a lot of harm, and Ichigo is determined to stop him.
A powerful sorcerer could be the main antagonist. This sorcerer might be after the same power source that Ichigo and Jackie are either protecting or trying to use for good. The sorcerer could be using dark magic to create chaos in their world and trying to stop Ichigo and Jackie from achieving their goals.