SPOILER!!!!!!!!!!!!!!
I don't know why but I have a crazy theory that Sirius is, in fact, alive. I think that maybe the deathly hallows are through the veil in the Ministry of magic, and Harry will ultimatly go through, and Find Sirius there, alive and well.
We never actually saw him die. We only saw him get hit with the curse and fall through the veil, but a body was never found. Given Rowling's track record for narrative misdirection, I feel that we haven't seen the end of Sirius. at least I hope not.
"I'm an American, Damnit!!! And if it's three things I don't believe in, it's quitting and math."