Ok, here is what I think. Sirius will come back, not dead, not alive, but as a dementor. Behind the viel could be the place where dementors are made, and that could explain why JKR would be so upset about Sirius's death, because she is going to make him into a dementor. That is the only reason why he wouldn't come back as a ghost or something around that area. Also, if Sirius becomes a dementor, he will obviously crush Harry. Nearly Headless Neck said he wouldn't come back as a ghost, they say he is dead, but he is just a dementor.