I have not posted any stories before, but have written a lot just to amuse myself. Most often they involve my wife or other people I know, but some have been based on existing characters.
One topic I have considered a lot is Harry Potter, or rather Hermione, since I think she is pretty hot. I only found a couple stories here using those characters. One uses the correct names while another changes them all slightly. Part of what I like about using a magical world is the lack of the usual boundaries. In the world of Harry Potter people would do all kinds of things. No one would get breast implants because they could use a spell and have breasts of a different size any time they wanted. The nerdy boy who could never ask a girl out might learn to make his back so flexible he can suck his own dick. For me, the reason to write about such a world is to explore things that just cannot be done in a normal world.
If you are interested in that sort of story, would you prefer them to involve well known characters or new ones, or perhaps just minor name changes? Are there copyright issues if the same character names are used? I assume not, since there is obviously no intent to pass it off as original work by the same author.