One idea could be that Naruto discovers a new ancient power source that fuses with his existing chakra, gradually transforming him into an 11 - tailed beast. This new form could give him abilities beyond what we've seen in the original series, like the power to manipulate time on a small scale. Maybe he uses this new power to go back in time and prevent some of the tragedies that occurred in the Naruto world.
One interesting fanfiction could be where Naruto has an unexpected and more powerful tailed beast transformation during a major battle. Maybe he combines the power of all the tailed beasts in a new way, surprising his enemies and allies alike.
Well, Naruto himself might still be a main character as he has a strong connection to the tailed beasts in the original series. He could be the one trying to understand and deal with the 10 - tailed beast in new ways.
In this fanfiction, the plot might involve Naruto being experimented on by some dark force. This experiment causes him to transform into a tailed beast in a way that's different from his normal jinchuriki forms. As he changes, he starts to view the world from a more primal perspective. He might even break away from the Hidden Leaf Village and the story could be about his journey to regain his sanity and his place among his friends and the village.
One interesting plot could be the exploration of the power dynamics when the 10 - tailed beast interacts with different ninja clans. Maybe it shows how they try to either control it or form alliances with those who can.
It could also change the way ninjas train. If the 11th tailed beast has a unique type of chakra, ninjas might start to develop new jutsu or training methods to adapt to this new power source. Maybe some ninjas would try to bond with it, creating new jinchuriki with different abilities compared to the traditional ones. And this would completely shift the balance of power in battles and the overall ninja society.
A creative use could be when Naruto uses the Tailed Beast Bomb to create a shield. By reversing the energy flow, he can form a protective barrier made of the same power as the bomb. This is unexpected as the bomb is usually seen as an offensive weapon.
There are some fanfictions that explore the Tailed Beast Bomb in a more strategic sense. For example, a story where Naruto has to use the Tailed Beast Bomb in a very limited space without causing too much collateral damage. This makes him come up with creative ways to control the blast radius and the power output, which is really interesting to read.
One idea could be the discovery of an 11th tailed beast that has been hidden away for centuries. The main characters in Naruto then have to figure out how to deal with this new and powerful entity. Maybe it has a unique ability that disrupts the existing balance of chakra in the ninja world.
It could be Orochimaru. He has always been interested in power and the secrets of the tailed beasts. He might see Naruto's transformation into a tailed beast as an opportunity to study and use that power for his own evil purposes. He could try to manipulate Naruto in his new state, perhaps by using genjutsu or other underhanded tactics.