There's a fanfiction called 'Naruto and Gin's Cross - Paths'. It focuses on the initial meeting of Naruto and Gin Ichimaru. The author does a great job of showing how their different moral compasses clash at first. But as the story progresses, they start to understand and respect each other, leading to a series of adventures that are both exciting and thought - provoking.
You can start by looking on fanfiction.net. It has a large collection of fan - made stories, including many 'Naruto Gin Ichimaru' ones. Just use the search bar and type in the relevant keywords, and you'll be presented with a list of available fics.
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.
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.
There might be a fanfiction where Ichimaru is actually a long - lost relative of one of the main clans in Naruto. For example, if he were related to the Uchiha clan, it could create a whole new dynamic. He might have a different Sharingan ability that he shares with Sasuke, and this could lead to them teaming up against a common enemy in a way that wasn't possible in the original series.
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.