My Theory: River Song and Amy Pond are the same person. They have been trying to mislead us that River is actually a Time Lord, but I think that is just a red herring.
I have a few reasons for believing this. In no particular order....
- River and Pond are both aquatic names
- Amy's job before traveling with the doctor was doing kissograms. River uses hallucinogenic lipstick to get out of trouble (or in, depending on how you look at it).
- River asks Donna who she is, but doesn't ask Amy when they first meet.
- River gives Amy the blank journal to trigger her memories of the Doctor. When she remembers him, he returns. They didn't show the pages filling in and why would River's journal trigger Amy's memories. Amy now has a blank journal that is later owned by River.
- River's history is often out of order. She is imprisoned in the 54th century when she leaves for the Pandorica. However, later when she meets them earlier for the battle with the Weeping Angels, she was imprisoned centuries before. This clearly shows that she doesn't have a set time that she resides in.
- River tells the Doctor that he will soon know who she is and it will change everything. Amy being River would fall into that category.
- River is imprisoned for killing "A very good man. Best man I've ever known". Possibly Rory?
- Amy and River are both feisty, outspoken and speak to the Doctor in the same tone.
- Amy and River both work together to destroy the Fez happily
- Last, but not least, my fellow Doctor Who fan, Shelby, noticed they both wear the same watch (as seen below).
This is my theory and I'll be curious to see how it all plays out. However, if I am wrong, I trust that the show will continue to be awesome.