Yes, there might be. Fanfiction websites often have a wide variety of creative takes on different characters. You could start by checking popular fanfiction platforms like FanFiction.net or Archive of Our Own. Just search for 'Naruto is the Fox Sin of Greed' in their search bars and see what pops up.
One possible plot could be that Naruto, in this fanfiction, has his greed manifested in his never - ending quest for stronger ninja skills. He becomes so obsessed with being the best and having the most powerful techniques that he starts to alienate his friends. His greed could also lead him to make deals with shadowy characters or organizations in the ninja world. As the Fox Sin of Greed, he might be hunted by those who oppose the sins or be part of a group of characters who each represent a different sin, and they have to either work together or against each other depending on their motives. This could lead to a complex web of relationships and conflicts as Naruto tries to either embrace or overcome his new identity as the Fox Sin of Greed.
The main characters could include the fox, which is the focus due to the 'fox sin of greed' part. Naruto is almost certain to be involved as well. Additionally, there could be some original characters created specifically for this fanfiction. These new characters might be related to the source of the fox's greed, like a mysterious entity that tempts the fox with offers of power or wealth. Their presence would add more depth and complexity to the story, as they could manipulate the fox's greed and create new challenges for Naruto and his comrades to overcome.
I don't think so. There haven't been any official announcements or indications that the HxH manga will continue. But you never know, maybe there'll be a surprise in the future.