Unlike certain other folk, I don't quite get fan fiction. Surely the enjoyment of fiction by only the author is that, like life, it's out of your control. Perhaps part of it is the mental challenge of writing within a constraint. Or, um, without any constraints at all, in certain cases.
Still, if Little Johnny has had all four books read to him a dozen times a-piece and is simply screaming for more, I suppose you could do worse than Diagon Alley, a Harry Potter fan fiction repository. But take care you stop reading before A Weather of the Heart, in which Harry is finally brought to his knees. In a most literal sense.