They would probably be shocked. Since Naruto is such an important figure to them, any big secret would likely cause a stir. For example, if it's about his Nine - Tails secret, they might be initially stunned as they realize how much he has been carrying alone.
Perhaps he reveals his dreams for the future of the Hidden Leaf Village that are more personal and not part of his official Hokage - related goals. For example, he might talk about his hopes for how the different ninja clans interact and co - exist in a more harmonious way, something that he has been thinking about but hasn't shared with the group yet.
Well, one possibility is that his friends might be initially shocked. But then they would likely accept him and vow to protect his secret. After all, they've been through a lot together already.
Kakashi could be involved too. Given that Kakashi has trained Naruto and seen his growth, Naruto might trust him with his secrets. He could reveal his thoughts on how he wants to change the ninja world for the better and the steps he is planning to take in the future.
In the fanfiction, Naruto's enemies' reactions could vary greatly. For instance, Orochimaru might see it as a chance to conduct more experiments on Naruto or the tailed beast. The Akatsuki, on the other hand, might change their plans completely. They were after the tailed beasts, and now that Naruto has revealed his, they might either try to capture him right away or come up with a more elaborate plan. And then there are smaller enemies who might just cower in fear at the thought of facing a jinchuriki like Naruto who has openly shown his power.
Another possibility is that he reveals secrets about his parents. He could talk about things his father, Minato, told him in a dream or some information he discovered about his mother, Kushina, that no one else knows. For example, a secret mission they were involved in or a special bond they had with certain characters in the Hidden Leaf Village.
One possible significance could be a sign of his growth and the battles he has endured. His scars are like a visual record of his journey, showing how he has overcome difficulties. They might also be a way to connect with other characters on a deeper level, as they can see the physical toll his life has taken.
Another possibility is that it could be a part of a self - discovery arc in the fanfiction. Maybe Naruto starts to question his own identity and through various interactions and experiences, he decides to reveal his true gender as a way to be more true to himself.
It's possible he gains enhanced physical abilities like super strength or speed. Since his heritage is often tied to some special genetic traits, these could be unlocked upon the revelation. For example, if his heritage is linked to a powerful clan, he could inherit their natural combat prowess. He could also find out about special chakra control abilities that are exclusive to his bloodline, which would give him an edge in battles.
Some characters might be shocked and saddened. They may have known Naruto as a strong and energetic person, and seeing his scars would make them realize the hardships he has faced.