Perhaps there's a prophecy in the fanfiction that Naruto is destined to become an 11 - tailed beast. As he starts to transform, he has visions of a future where he is either a great savior or a great destroyer. This internal struggle drives the plot. He tries to find a way to balance the power of the 11 tails without losing himself. Along the way, he meets new characters who have knowledge about the 11 - tailed beasts and they guide him on his journey.
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.