River Pond Song is Amy's daughter: a River Comes from a Pond.
The Doctor, by meeting Amy, has prevented her marriage to Rory thus prevented River being born.
We know that the Doctor encounters River at a place called "the Pandorica", and that she survives whatever Bad Thing happens there. Presumably River contributes in some way to the Doctor's victory.
So by meeting Amy and preventing River coming into existence, the Doctor has created a paradox which has brought about a space time split fracture crack thingy.
I assume that River is Amy's daughter by Rory: but it is possible that she is Amy's daughter by the Doctor. This would account for River's Time Lordyness. A lot of her flirting could be "grown up daughter" flirting rather than "wife" flirting.
But even if this is right, Amy's wedding day is still the epicentre of the paradox. What breaks time and space is Amy marrying Rory, as opposed to Amy not marrying Rory: the very actions which the Doctor is taking to prevent the paradox are, in fact, creating it.
Could it even be that the Crack is consciously removing the obstacles to the Doctor marrying / sleeping with Amy to bring River into existence and heal the wound in the universe? For example, it might be said to have dragged the TARDIS back in time 14 years in the episode one; and deliberately erased Rory from existence in the one with the Silurians.
I am somewhat afraid that we are going to be told that large lumps of continuity have also disappeared into the Crack; or even that the whole universe will be be sucked into the Crack and that Steven will say that, as of next year, the series is taking place in a new, post-Crack Doctor Who Universe to which the history of the pre-Crack Universe no longer applies. I hope not. It seems to me that if Doctor Who fans have spent 50 years happily believing that six impossible and mutually contradictory things happened in the same "universe" before breakfast; there's no reason to think we need a Big Continuity Clear Our this late in the day. But note that Moffat wanted to refer to Season 5 as "Season 1".
There's no reason that The Crack shouldn't be left lying around for fans to use as a hand wave. "The Romana regeneration doesn't make a lot of sense in the light of what we know about other regenerations, does it?" "Oh well, let's just assume it disappeared into the Crack."
Even before the trailer (and isn't Steven being good at keeping the trailers mostly spoiler free) we could probably tell that "The Pandorica", which is going to open, would be a box of some kind. There was a box in Greek mythology with a similar name. It contained something very horrible, I seem to remember.
Some people would like it to contain something Time Lordy: the Skaro Abominations or the Could Have Been King or something else that was referenced in The End of Time. Rassilon himself, maybe, or the whole of Gallifrey. I would cast my bet against "The Other" or "Omega": these are characters who only fans know about, and significant recurring villains in the new show have to be in the consciousness of the mainstream public. I can't believe that Moffat would be so boring as to make it the Daleks or the Master.
The Box is not the Crack, but opening the Box obviously has to be closely related to whatever caused the Crack. Several villains have implied that the contents of the box is obvious and it's funny that the Doctor doesn't know. The specific phrase "The Doctor in the TARDIS" has come up twice.
So my money is on the Pandorica containing an evil future incarnation of the Doctor. Fans will be able to say "The Valeyard" to their hearts content, but he won't be called that on screen, or only in passing. Seasons 1 – 4 kept on telling us that the Doctor would turn evil one day; and we've had the Toby Jones anti-Doctor to lay the groundwork.
Who will be playing the evil version of the Doctor?
I can't believe that this wouldn't have leaked out, but I do have to point out that in the one in the flat the Doctor becomes Craig's lodger. A person who pays him money to use a section of his property.
That is to say, a tenant. I've been wrong before, of course.
continues....