The 'juubi naruto sealed in his younger self' concept shakes up the Naruto universe in fanfiction. Since Naruto has the Juubi sealed within him at a young age, it would affect his relationships. His friends might be in awe of his new power or might be a bit scared. His teachers would have to train him differently. Also, the political situation in the ninja world would be different. Konoha might become a more dominant force, but at the same time, it could also attract more powerful enemies. And Naruto himself would have to struggle with the moral implications of having such a powerful entity within him at a young age, which would add a whole new layer of depth to the fanfiction story.
In fanfiction, this concept would have a huge impact. Naruto's growth would be different. He would have to deal with the power of the Juubi at a time when he is still developing his own identity. This could lead to a different Naruto, one who is more mature in some ways and yet still retains his naivete in others. The story could also explore how the village and its people react to this new Naruto. Some might fear him, while others might see him as a new hope. And in battles, Naruto could potentially defeat enemies much quicker, which would change the overall plot of the Naruto universe in fanfiction.