It's hard to say for sure. Both Aizen and Yhwach are incredibly powerful characters with unique abilities, and it could come down to a matter of strategy and circumstances on the battlefield.
One common theme is Kazui's discovery of his powers. Since he is a new and somewhat mysterious character, many fanfics explore how he comes to understand and control his abilities, much like Ichigo did in the original 'Bleach' story.
One interesting Kazui Kurosaki fanfiction could be about his journey in developing his own unique Shinigami powers. Maybe he discovers a new form of Zanpakuto that has powers different from his father's.
One popular character paired with Kazui Kurosaki in fanfiction is Ichigo Kurosaki, his father. Writers often explore their father - son relationship in different ways, like Kazui learning new skills from Ichigo.
The plot could involve the Vizards, with their unique powers and experiences, encountering Kazui Kurosaki. Maybe they sense his latent power and decide to train him, leading to a series of adventures where Kazui learns to control and expand his abilities under their guidance.
You can try looking on fanfiction websites like Archive of Our Own (AO3). There are often a lot of fanfics related to various 'Bleach' characters, including Kazui. Just use the search function on the site and type in 'Bleach Kazui' to see what pops up.
Kazui Kurosaki is bound to be a main character. He might be a young and inexperienced but full of potential. The Vizards, as a collective or perhaps some specific members among them like Hiyori Sarugaki or Shinji Hirako, would also be main characters. Hiyori could be the tough - love type of mentor to Kazui, while Shinji could offer more strategic guidance in their adventures together.
I can't help you. If you have any other questions or needs, please let me know. I'll do my best to answer them.
While waiting for the TV series, he could also click on the link below to read the classic original work of " The Legend of Mortal Cultivation "!