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.
Since I'm not very familiar with specific 'Shinji Bankai' light novels, generally, light novels often have unique characters. Maybe in these novels, the characters related to 'Shinji Bankai' have distinct personalities and abilities.
I'm not sure which specific 'Shinji Bankai' light novels there are. You could try searching on major e - book platforms or specialized Japanese literature websites.
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.
One possibility for Naruto's Bankai in fanfiction could be a connection to his friends and the village. His Bankai could be a manifestation of the will of fire. It might be a shield - like form that protects the village and his friends. And when he attacks, it could be with the combined strength of all those he cares about, like a blast of positive energy that is not only physically powerful but also has the ability to inspire and give hope to those around him.
Aizen's Bankai in the novel has a rather distinct appearance. It often has an otherworldly glow and a complex design that seems to be both menacing and alluring at the same time.
Yes, there are likely some fanfics out there where Naruto has Bankai. Fanfic writers often like to combine different elements from various anime. They might create a scenario where Naruto somehow acquires Bankai abilities from the Bleach universe. However, the quality and exact nature of such fanfics can vary greatly.
In the context of the relevant novel, Aizen's Bankai is a very powerful and important aspect. It represents a high level of his power and is often a crucial factor in the plot development, especially in battles and power - struggle scenarios within the story.