Sadly, I don't have a list of 'Harry has a secret child' fanfictions to recommend. But when you search for them, pay attention to the ratings and reviews. Fanfictions with high ratings and positive reviews are more likely to be well - written. Also, consider looking at the works of popular Harry Potter fanfiction authors, as they might have written something similar in the past.
Sure. There could be a fanfic where the rad child has the power to control time in a small way. They can slow it down during a Quidditch match to ensure their team wins, or speed it up during a boring class. This unique ability sets them apart and leads to all sorts of adventures and challenges at Hogwarts.
It could be Ginny. Even though in the original story they are married and have kids in a more normal way, in fanfiction, there could be a twist where their relationship had a secret earlier on and she is the mother of the secret child.
I'm not sure of exact fanfictions similar to it. But you could look for other Harry Potter - Naruto crossovers on fanfiction websites. There might be ones where Harry has some sort of connection to other Naruto clans too.
There might be 'Harry's Adventure in an Alternate Magic World' which, like 'harry is a meister fanfiction', gives Harry a whole new world to explore and adapt to. It could have similar elements of him having to learn new ways of using magic and interacting with different characters.
You could also try searching for fanfictions where the setting, like the sky in this case, plays a major role in the character's development. Maybe a story where a character is on a high mountain all the time and the view of the sky above affects their thoughts and actions, much like how the sky affects Harry in the 'harry is a sky fanfiction'.
One similar fanfiction could be 'Harry's Alternate Journeys'. It also takes Harry on different paths and adventures that are not part of the main canon. Another one is 'A General Harry in a New World' which has a similar concept of a more general and versatile Harry in a different setting.
One could be 'Hermione's New Beginnings'. It has a similar focus on Hermione's personal growth and relationships, although not specifically centered around a child. Another is 'Hermione's Uncharted Path' which also explores Hermione in different situations and relationships that are somewhat parallel to the themes in 'For Love of a Child'.
There's 'The Redemption of Severus Snape' fanfiction. It often involves a lot of emotional turmoil and difficult situations for the characters, much like what could be in a fanfiction about pain. It focuses on Snape's journey which is full of challenges and pain, just like Harry might face in the 'six paths of pain' one.
One possible plotline could be the discovery of the secret child and how it affects Harry's relationships with his friends and family. For example, Hermione and Ron might be shocked at first but then become supportive. Another could be the challenges the child faces growing up in the wizarding world with this secret hanging over them.
I'm not sure of a specific one off - hand, but you can try searching on fanfiction websites. Look for ones with high ratings and lots of reviews.