*bangs head against wall* Sirius will not come back in physical form. Nor will he come back as a ghost. But what Luna said makes things confusing. What's beyond the Archway is obvious: Rowling's Verison of Heaven for Wizard's and Witches, except it's something more like Field of Dreams thing. (I'll explain later) "Oh they were just lurking out of sight, that's all"- Luna. Echos of the dead?
I agree about the mirror that Harry had, something most likely will happen in the 6th book to do with sirius and harry. In the wizarding world, if you die you come back as ghosts or some other form so maybe harry will meet him and quite possibly his mother and father in the seventh book during a collision in time, that would actually be quite cool, don't you think?
Luna might know something about Sirius that we don't know
N XX π π π π π
ha ha found it yay
1) Q why did you kill Sirius? It made me very sad
A I'm really, really sorry. I didn't want to do it, but there was a reason. If you think you can forgive me, keep reading, you'll find out. [i feel really guilty now].
2) Q If we ever see Sirius again, what form will he be in?
A I couldn't possibly answer that for fear of incriminating myself.
Hope that kinda helps! although feel free to disagree...