River Song is probably the most interesting recurring character in the Whoniverse. I won't bother to explain all about her here, if you watch the show you know. If you don't watch the show, you don't care, so let's just assume you follow it.
There's much speculation about who and what she 'really' is, and her role in the Doctor's life. Personally, I think it's fairly straightforward: The Doctor's first meeting with River is her death. Her first meeting with him is his death. Basically it's just a device to give the Doctor a more interesting character to play off of outside of the standard "Companion" thing.
I don't think she's his mom/sister/daughter/self/the rani/future amy/ or any of the other hoo-hah that's been suggested without any real evidence. Certainly he'd know if she was a Time Lady, since Galifreyans can all sense each other. I believe she's exactly what she says she is - which is plenty interesting enough on its own - she's an amoral, murderous, stalkery woman who somehow got him to marry her. Do we need more than that?
It's strongly implied that she deliberately brought about his death. "Death" could mean regeneration, but I don't think so. That'd be too cheap, at least under normal circumstances. Obviously it can't *really* be the Doctor's death, but just as clearly River tries to kill him for some reason, and she believes she's succeeded. Not only that, the authorities believe so as well, as per the priest/warriors (I really, really, really liked them, btw, particularly the Bishop's last scene. Wow!).
Thus at some point in the Doctor's future he comes close enough to death that people think he's dead for a substantial period of time.
The only real questions here is *why* does she kill him, and why does she think he's really dead? On their first meeting? And then spend a lifetime trying to make it up to him? Certainly the authorities seem none too pleased.
It seems to me that this is the kind of thing they'll throw in as a regeneration story, like when Smith decides to leave. But will they? The rumor is that Smith is a short-termer: Two years and done. That would be a waste, given her implied loooooong history with the Doctor (Which we mostly haven't seen) and I like River and would like to see her for a good while longer.
And I suspect we'll see her for several Doctors more. Why? She thinks she kills him because whatever it is she does is at the end of his 13th life!
HA! Think about it: Why cast a middle-aged actress in the standard hot-chick costar role? Presumably, given his tastes, she *was* a standard hot chick costar when they met, and she's grown old over time. This implies half a lifetime together, which fits with her own statements somewhat. If so, I'd expect her to be recast at some point in the not-too-distant future.
So she really thinks she's killed him, and everyone else would too under the circumstances. And in fact, the Doctor himself probably would really think he's dying as well.
Hm. She's an Archeologist. We know the Time Lords could resurrect people, or bestow additional lives as a reward in certain circumstances. What if the 13th Doctor needed an archeologist to try and find some long-lost Galifreyan device whereby they did this? Something that wasn't on the homeworld when it was Time Locked. Say, a prototype rumored to have been lost centuries before the war while being shipped to a museum or captured by the Sontarans during their time on Galifrey or whatever. Thus River not only kills him, but also provides for the rejuvination of the show. That's actually a fate befitting a character as large as her, yes?
So "You will see me one more time" simply refers to Smith's Doctor, who, I reiterate, is rumored to be a short-timer: Two and Done. (I hope Amy stays on, though. R2's got a much-deserved crush on her.)
So that's my theory. It'll take a half decade or more to find out if I'm right, of course, but I figured I'd file it away here in public record so I can say "I was right" when the time comes.
Sound off: how mistaken am I?