I'd suggest checking manga forums or dedicated Bleach fan sites. They often have detailed breakdowns and recommendations on where to pick up after a specific part. Sometimes, the author or official sources might provide cues as well.
Honestly, I don't know when the TYBW manga starts. But you could search manga databases or ask in manga enthusiast communities to get the details you need.
One interesting concept could be the unique character interpretations. In fanfics, authors often bring new perspectives to the existing characters from the original source that 'and the beast cometh' is based on. For example, they might explore the backstories of minor characters more deeply, or give a different personality trait to the main characters which can make the story more engaging and complex.
First, know the source material well. Understand the characters, their motives, and the world they live in. This will help you write a fanfic that stays true to the essence of 'and the beast cometh'.