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.
You can start by looking on fanfiction.net. It has a large collection of various fanfictions, and there's a good chance you'll find some Harry is Unohana fanfictions there. Just use the search bar and type in the relevant keywords.
Well, there's a fanfic called 'Unohana's Influence on Harry'. It shows how Unohana's calm and collected nature starts to rub off on Harry. He becomes more in control of his emotions, especially his anger. Then there's 'The Bond Between Harry and Unohana' which focuses on the deep emotional bond that forms between them as they share their life stories and experiences. There's also 'Harry in Unohana's World' where Harry is transported to Unohana's world and has to adapt to the different rules and customs, all while relying on Unohana for guidance.
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.
One common theme is the exploration of an unlikely relationship. Since one is an established character (retsu unohana) and the other is an original creation (oc), the fanfic often delves into how they meet and interact. Another theme could be the oc's growth with the influence of unohana. For example, the oc might learn about healing or combat skills from her.