Naruto's bankai might incorporate his fox - like features. Given his connection to the Nine - Tails, his bankai could have a large, fiery fox form that enhances his speed and strength, much like how his Nine - Tails Chakra Mode works. His zanpakuto could also be related to the rasengan, perhaps having the ability to fire off rasengan - like attacks from the blade or using the spinning energy as a defensive mechanism.
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.
In a Naruto achieves bankai fanfiction, it could happen through some sort of cross - over magic. Maybe a powerful jutsu misfires and transports Naruto to the Bleach universe where he somehow gains the ability to achieve bankai. It could also be due to a powerful being bestowing this power upon him in an attempt to create a powerful ally or as part of a grand experiment.
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.