Still when a writer leave any threads unfinished they leave themselves open to someone being disgusted with them for leaving them hanging.
Yet I've read a few novels where it's just the one novel and the author decided to leave it up to the reader to imagine what's going to happen next.
I don't think I'd be disgusted with an author for not tying up every single thread completely and leaving no room for speculation. Indeed, leaving some things open to the audience is actually better in a lot of ways- and on top of that, it does make it easier to continue the story later. There should still be at least some closure, though. In a sense, it's a difference between "what happens" vs "what happens next".