In the manga, Senjumaru did not employ Bankai. The character's abilities and actions were depicted differently without resorting to this particular power.
One interesting 'harry potter bankai fanfiction' could be where Harry discovers a hidden power within himself that combines the magic of Hogwarts with the concept of bankai from 'Bleach'. For example, he might use it to defeat Voldemort in a more spectacular way than in the original series.
Shinji did use his Bankai in the manga. It was a crucial moment that turned the tide of the fight. The Bankai's power and unique abilities were showcased, adding an exciting element to the story.
The Bleach manga faced some challenges in terms of maintaining its popularity towards the end. However, it still left a significant mark on the manga industry and has a dedicated fan base. The ending was met with mixed reactions from readers.
In my opinion, both have their merits. The anime brings the characters to life with animation and sound, but the manga allows for more imagination and detail in the panels. It really comes down to what you value more in your entertainment.
Another way is through character development. A character in Harry Potter, say Neville, could go on a journey to master a bankai - like power. This could involve him training in a special place, like the Room of Requirement, which could transform into a training ground similar to the ones in 'Bleach' for bankai training. He would face challenges and grow as a character, all while blending the two concepts.