Obviously, Ichigo is the central character in 'ichigo goes beyond bankai fanfiction'. But we can't forget about his mentor figures like Urahara. Urahara has always been a bit of a mystery and has a lot of knowledge about spiritual powers. He could be crucial in helping Ichigo understand and master his new powers. Then there are Ichigo's opponents. There might be new and powerful adversaries who are either threatened by Ichigo's new power or are trying to harness it for themselves. These opponents would also be important characters in the story.
One interesting plot point could be Ichigo's new powers and how they change his battles. For example, he might have abilities that allow him to manipulate time or space in combat. Another could be the reactions of his friends and foes to this new level of power. His friends might be in awe or a bit worried about his newfound strength, while his enemies would likely be more fearful and have to come up with new strategies to face him.
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 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.
Yes, in some fanfictions, Naruto is given a Bankai. Fanfiction writers often like to cross - over different anime concepts. They might give Naruto the power of a Bankai from 'Bleach' to create unique and interesting storylines. This can lead to some really cool battles and character developments that you wouldn't see in the original Naruto series.
In fanfics, anything is possible. Since it's a fictional creation, an author could write a scenario where Naruto has the ability to use Bankai. However, in the original Naruto and Bleach universes, their powers are completely separate, with Naruto having chakra - based ninja abilities and Bankai being a high - level power in the Bleach world.
In the manga, Senjumaru did not employ Bankai. The character's abilities and actions were depicted differently without resorting to this particular power.