I believe River Song is...the Spaceman who kills the Doctor in the future, and Rory and Amy's daughter.

Think about it.
A) If you reverse River Song, you get Song River which is a similiar name to Amy Pond. They share some personality traits like a free spirit and a fiery nature.
B) The Cleric said to the Doctor in Flesh and Stone that River was in prison for killing a great man. I believe that great man was the Doctor and the Clerics somehow found out what River did in her past and didn't approve it at all, probably having zero tolerance for any crimes, no matter how far in the past they are.
C) I also think she may remember her past as the Spaceman and that's why she told Amy and Rory to keep it from the Doctor because she may have been afraid that the Doctor may have figure out that she was the Spaceman and misinterpret what she did to his future self. Remember how the future Doctor already knew who the Spaceman was on the spot and doesn't seem that afraid of her. I also think Amy missed hitting the Spaceman/future self with the bullets from her gun on purpose, to make it appear that she wants "it" dead in front of an emotionally distraught Amy and Rory.
D) And, since she is a Time Lady or whatever like her past self, that would explain why the Doctor has some much trust in her because of their similiar natures.

Think about it.
A) If you reverse River Song, you get Song River which is a similiar name to Amy Pond. They share some personality traits like a free spirit and a fiery nature.
B) The Cleric said to the Doctor in Flesh and Stone that River was in prison for killing a great man. I believe that great man was the Doctor and the Clerics somehow found out what River did in her past and didn't approve it at all, probably having zero tolerance for any crimes, no matter how far in the past they are.
C) I also think she may remember her past as the Spaceman and that's why she told Amy and Rory to keep it from the Doctor because she may have been afraid that the Doctor may have figure out that she was the Spaceman and misinterpret what she did to his future self. Remember how the future Doctor already knew who the Spaceman was on the spot and doesn't seem that afraid of her. I also think Amy missed hitting the Spaceman/future self with the bullets from her gun on purpose, to make it appear that she wants "it" dead in front of an emotionally distraught Amy and Rory.
D) And, since she is a Time Lady or whatever like her past self, that would explain why the Doctor has some much trust in her because of their similiar natures.