I think JKR knows enough not to write another HP sequel or spin-off. I do hope she writes that encyclopedia she's talked about doing, and I wish she would write a Marauders era prequel series, but that's just wishful thinking. As much as I would love more books, it would be a bad idea to write more about Harry and co. The epic part of his story has been told; there's nowhere to go that would be as good as the original seven books.
It's so easy to get sucked into writing series, especially when you're just beginning and don't know any better. That's when I wrote most of my sequels too. I don't even know why; it's not like I was getting enough feedback in the first place to make me think that writing a sequel would get me more, and I definitely didn't suffer from a lack of ideas. I think most of my sequels were written just because I enjoyed writing the first story and wanted to keep it going, and others were written (or started and never finished) because I got sick of writing the first story and decided to end it early and save the rest for a sequel. Bad idea! The last novel sequel I wrote was BMS, and that's the only one I don't regret writing, although obviously there are parts I wish I had written differently.
