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 Naruto fullbring fanfiction could be one where Naruto discovers a new form of fullbring that combines his chakra powers with it in a unique way. For example, he might be able to fullbring out the essence of the Nine - Tails in a more controlled and powerful form.
Naruto himself is an obvious choice. His ability to adapt and his large amount of chakra could lead to some really cool fullbring combinations. He could bring out new and powerful forms that we've never seen before.
One possible plotline could be Naruto discovers he has a hidden Fullbring ability. He then has to learn to control it while still dealing with the threats in his own world. Maybe he uses it to fight stronger enemies like Madara in a new and unexpected way.
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.