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.
One common theme is power. Yamamoto is extremely powerful, and fanfics often explore the extent of his power, like how he can handle really tough enemies. Another theme is leadership. As the head of the Gotei 13, his leadership style and decisions are often the focus. For example, how he manages to keep the Soul Society in order.
There's a fanfiction called 'Yamamoto's Origins'. In this, the author explores Yamamoto's youth in great detail. It includes his training, his relationships with other characters back then, and how he came to develop his powerful abilities. It's a really interesting read for any 'Bleach' fan interested in Yamamoto's backstory.
One common theme could be power - level comparisons. Since Naruto and Bleach both have complex power systems, a fanfic might explore how Yamamoto's power from Bleach compares to the various powers in the Naruto universe.