I don't know for sure. But usually, such sites have guidelines to prevent inappropriate content. You could look for ratings or warnings on the specific fanfic to determine.
Another story could focus on Naruto's relationships with his friends as an adult. For example, how his bond with Sasuke has evolved over time and how they support each other in different ways. It could also show how they train the next generation of ninjas together.
Some popular fanfiction websites like Archive of Our Own (AO3) might have a good collection. They have a wide range of Naruto - related fanfictions, including those that focus on the characters as adults. You can search using relevant tags such as 'Naruto - Adult' or 'Naruto - Post - Shippuden' to narrow down your search.
Pretty much. Fanfic.net is known for being a place where most of the content is fanfiction. However, there might be a rare exception or two, but the vast majority of what you'll find there is related to fan-created stories based on popular franchises.
On Fanfiction.net, the Naruto fanfics about black baby could cover a wide range. Maybe there are ones focusing on their unique powers or their journey of growth.
Archive of Our Own (AO3) is a great alternative. It has a more inclusive and diverse range of fanfiction. It also has a better system for categorizing and tagging stories. Another alternative is Wattpad. While it has a mix of original and fan - based content, you can still find some good fanfiction there. Additionally, Tumblr can be a source of fanfiction, especially if you follow the right blogs.
Well, there could be fanfics that focus on their training sessions together or even alternate universe scenarios where their roles and powers are different. It really depends on the creativity of the authors on FanFiction.net.