Then it's the third real prediction she's made, isn't it?
I think at the end of this whole thing Sirius will finally be back. Our dear JKR probably wants to keep us in a shock to show us how evil Voldemort can be. Because we know Voldemort killed many people in the past, we may feel sorry for them, but we didn't really know them. There had to be someone who we meet, know, love and finally lost.
HP without Sirius is like LOTR without Aragorn or like Star Wars without Han Solo, isn't it?