My understanding is that the site will contain the Potter books with additional material Rowling left out of the original books. There will not be any more actual new material in the Potter universe. Hopefully, someday she'll continue the story or open the universe to other writers, but that isn't on the table right now. The site is simply a way for Rowling to distribute the Potter ebooks so that she can make the most profit without selling them through third party sites.
The life of the Potter franchise has been debated en nauseum here on the boards. I absolutely think it will be beloved for years to come but I also think its days are numbered as the huge draw it is now. I would love to see new content but unless that happens, it will eventually lose it's luster.
If Lucas had not made the prequels (and not saved the prequels by the new animated show
), I think Star Wars would have much, much less draw that it has now.
It's simply a matter of out of sight, out of mind.