I doubt that we'll find out on Saturday.
Yep, I don't think who or what River is will come out any time soon, it would spoil all the random little comments and things that are spattered around in the conversations she has with the Doctor.
Saturday will just be the Doctor (or Amy or Rory) pulling a rabbit out of a hat to somehow foil the Daleks, Cybermen, Sontarans etc, assuming Amy somehow survives or Rory manages to throw off the control of the Nestene Consciousness.
I suppose if the Tardis doesn't explode and create all the cracks in time, then anyone who disappeared because of the cracks wont, and could "reappear". This could bring Rory back. It would also bring all the Weeping Angels back, but I suspect that little story twist would be ignored!