Well, for a Bleach Halibel x OC fanfiction, a plot could revolve around the OC being a time - traveler. The OC travels back in time and meets a younger Halibel before she became an Espada. This could change the course of Halibel's life and their relationship. Or, the OC could be a musician in the human world whose music can reach the spiritual realms. Halibel hears this music and is drawn to the human world to find the source, which is the OC. Also, the OC could have a special ability to communicate with water, just like Halibel has an affinity with water - based attacks. They could bond over this shared connection.
One plot idea could be that Ichigo and Halibel are forced to team up against a new, powerful enemy that threatens both their worlds. They start off distrusting each other but gradually learn to respect and rely on one another's strengths.
To develop their relationship, start with small interactions. For example, the OC could save Halibel from a surprise attack by other Hollows. This would make Halibel curious about the OC. Then, have them exchange stories about their lives. Halibel could talk about her rise to become an Espada and the OC could share their origin story. Another way is to give them a common goal, like protecting a group of innocent Hollows from a cruel Arrancar. Working together towards this goal would bring them closer.
One idea could be that the OC is a powerful healer who joins Zaraki's squad. Their different outlooks on battle and life create an interesting dynamic. Zaraki, being all about combat strength, initially doesn't understand the value of the OC's healing abilities. But as they face tougher enemies together, he starts to see how crucial the OC is to the survival of the squad. This leads to a slow - building relationship between them.
Well, you could have an OC Hollow who was once a human with a tragic past. After becoming a Hollow, they retain some of their human memories and emotions. This could lead to some interesting internal conflicts as they struggle between their Hollow instincts and their human side. Also, a story where an OC Hollow teams up with other Hollows to take on a powerful common enemy, like a rogue group of Soul Reapers who are experimenting on Hollows in cruel ways.
A really interesting concept could be a time - travel based 'halibel x ichgo fanfiction'. Ichigo somehow gets sent to Halibel's past or vice versa. They have to navigate this new situation together, with Ichigo introducing new ideas to Halibel's world or her learning about his future. This could change the course of their relationship and the events in their respective universes.
One idea could be an OC who is a new transfer student at the high school in the Bleach universe. They have a hidden power that starts to surface when they encounter Hollows on school grounds. Another could be an OC who forms an unlikely friendship with a Soul Reaper in disguise at the high school and together they face off against a powerful Menos Grande that has targeted the school.
Maybe in the fanfiction, Ichigo gets lost in Hueco Mundo and Halibel finds him. Instead of attacking, she decides to help him find his way back. During their journey, they encounter various challenges and secrets about Hueco Mundo are revealed to Ichigo through Halibel. This could lead to a new understanding between the two species they represent.
Well, you could create an Arrancar OC that was once a human with a strong sense of justice. After becoming an Arrancar, they struggle with their newfound powers and the need to follow Aizen's orders. They might secretly help the Soul Reapers in some ways. Or perhaps an Arrancar OC with a power that is based on sound. They can use high - pitched frequencies to shatter objects or low - pitched ones to paralyze opponents. Also, an Arrancar who is an outcast among their own kind because they have a different view on the war between Soul Reapers and Arrancars could make for an interesting fanfiction.
Well, you could make an OC Quincy who is trying to bridge the gap between the Quincy and the Soul Reapers. This Quincy could have befriended a Soul Reaper during a mission and now wants to promote peace between the two groups. Maybe they start by sharing their Quincy knowledge with the Soul Reapers about how to better defend against Hollows in a non - violent way. And also, they could learn from the Soul Reapers about their way of maintaining the balance in the afterlife.
One idea could be creating an OC who has the power to manipulate time in the Bleach universe. This OC could use their time - manipulation abilities to prevent certain tragic events from happening, like the death of a beloved character. Another interesting concept is an OC with the power to absorb and then redistribute spiritual energy in unique ways, perhaps creating new and powerful attacks.