Yes, there are likely many fanfictions out there with the concept of Harry Potter having a secret wife. Fanfiction writers often come up with creative and alternative storylines to the original Harry Potter series. Some might explore this idea to add new dimensions to Harry's character or create a whole new plot within the wizarding world.
Well, Neville could be a significant character. He has come a long way since his Hogwarts days and could be a source of inspiration and support for the secret child. Ginny, as Harry's wife, is also very likely to be in these fanfictions. She could be a protective mother figure, or she might be involved in the decision to keep the child's identity a secret in the first place. And of course, there could be new characters created specifically for the fanfiction that interact with the secret child in various ways.
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.
It could be a Muggle - born witch who he meets outside of Hogwarts during the summer holidays. She's not part of the typical wizarding society he's used to, so they keep it a secret as it's an unusual situation for a wizard like Harry to be involved with a Muggle - born in such a way.
One interesting idea could be the exploration of how the secret child discovers their magical heritage. For example, they might start showing accidental magic and then begin to piece together their connection to Harry Potter. Another idea could be the child facing unique challenges in the wizarding world that are different from what Harry faced, like dealing with the expectations of being Harry's child in a more modern and changed wizarding society.
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.
Perhaps the plot could be that the child of Harry Potter gets sent back in time accidentally. There, they meet a young Harry and his friends. The child has to be very careful not to disrupt the past too much while also trying to find a way back to their own time. This would be filled with lots of tricky situations, like when they almost reveal future events or when they have to avoid being seen by their own young parents.
Maybe the main villain could be a corrupted Hogwarts professor. This professor could be secretly teaching dark magic to students and when Harry's child discovers this, they become the target. The professor wants to stop the child from revealing their dark secrets and will go to great lengths to do so, making them a formidable villain in the fanfiction.
Well, if Harry has a kid with a Muggle in fanfiction, it would likely be a very interesting story. First off, the genetics would be a wild card. Magic genes might interact with Muggle genes in strange ways. Maybe the child has weaker magic or a completely different type of magic. Socially, it would stir up a lot of gossip and probably some prejudice in the wizarding community. But on the positive side, it could also bridge the gap between wizards and Muggles more. Harry, being a hero, would have to deal with all these issues while raising his child.