Having just finished the final episode of Doctor Who Season 5, I wanted to do some research into what theories the fans were spinning out for the identity and purpose of River Song.
SPOILERS TO FOLLOW - DO NOT READ IF YOU HAVE NOT WATCHED ALL OF THE DOCTOR WHO EPISODES AS OF NOVEMBER 5th, 2010!!!!!!!!!!!!
Here are 8 pages filled with theories I found interesting --- some are quite easily disproven and others are rather viable...
So what are the theories we see so far?
River is the Doctor's Mother or Daughter
-- eh, much too creepy.
River is Jack.
-- not possible. River dies in the first episode we meet her in, whereas Jack (as the Face of Boe) dies in "Gridlock." Plus it's also very, very creepy.
River is Romana, the Rani, Rose, Donna, another past companion
-- I really don't think this is likely AT ALL. Some companions (the Rani) have copyright restrictions, and others are very clearly completely wrapped up for good.
River is Amy somehow regenerated
-- Personally I really don't like this idea but I can't rule it out for good.
River is a future incarnation of the Doctor
-- No, because she wouldn't need the diary to recognize what time point they were meeting in, nor would she be surprised by the fact that the Doctor doesn't know her when they meet in the LIBRARY.
River is a future companion of the Doctor with her own history/game
-- more likely than anything on this list.
My personal favorite theory? River is the TARDIS. I think that would be the coolest of anything previously mentioned. I don't think it's necessarily true as it is very farfetched, but it would be by far the most interesting.
And then there is the perhaps bigger question. Is River the Doctor's Wife? His Murderer? Both? Neither?
What is the truth going to be? I don't know. What I do know is that after "The Pandorica Opens" and "The Big Bang" I actually like River now. I have to stop casting premature dislikes on characters in Doctor Who... (Rose, the Ninth Doctor, River Song) because I invariable change them...
Completely different side note. RORY ROCKS! 'Nough said.
Just can't stop thinking about River! Read my additional thoughts here: MORE River Song.