I’ve heard a loooot of ideas in Doctor Who forums! The one’s I found the most interesting are:

That she could be The Master, or the master wife (remember de ring being picked up at the end of season 3) and that would explain the screwdriver (he would have stolen it from The Doctor in the future) and her knowing so deeply about the time lords.

That she could be the girl! She said “hello sweetie” and her father keeps saying to her “sweetie”, maybe river song is someone created by the girl to save the Doctor and Donna. When the doctor searched for life forms in the computer, there where only 2, but moments later Doctor River Song appeared.

That she could be Jack. (Yeh I know Jack) Becaaaause, he travelled trough time with the doctor to the end of the universe (like she says in the trailer) and she does flirt a lot like the Jack! And if in fact Jack is the Face of Boe, we know he will get pregnant so he might be a girl before becoming the Face of Boe. Also he comes and goes of The Doctor’s life that’s way he has the book.

That she could be the Tardis! Like Astrid, she could be the human form of the Tardis, that would explain that specific book (it looks like the tardis), that in the trailer she said “I hate you sometimes” and the fact that she haaaas been with him to the end of the universe, and whay she has the screwdriver (if he would give it to anyway it would be her)!

For me I’m thinking she might just be a future Companion, like Jack coming and going, and that he give her A screwdriver not THE screwdriver (in season 2 he lets his screwdriver fall… and I’ve never understood if he has more screwdrivers or he just fixed his old one). I also think she knows the name Donna and probably Rose and Martha, because the doctor explained her what happen (if she in fact is going to betray the doctor), I also think she is going to die and tells the doctor that when she meets her not to tell her how she is going to die (because the name of this episode was going to be River's Run like Gabitha said in a pick)!
The only think that doesn’t fit is, that journal is quiiite big and she knows the ten, soooo is David Tennant going to be in the show for more than just this season or the next one?

Oh I think I’ve got get carried away =P

I think, (or at least hope) that she and the Doctor are siblings. Some of her teasing is a bit like that. Some of its not, but you never know, their aliens. I just really hope she's not love interest. I can't handle that till he regenerates, and I don't want that to happen for a long time.

I think there's a lot of conjecture about the screwdriver, yet we saw in an earlier episode another alien using a sonic screwdriver. I don't think they're that uncommon in the universe; it's just a variety of common tool that the Doctor likes to use. So it's not that extraordinary that he gave it away. That said, I think she's a future companion. I think that means that they have to hurry to explain the relationship before Tennant leaves the show, but my wife disagrees: she thinks it's quite plausible that she somehow recognized him in an earlier regeneration, despite her knowing him in a later regeneration. I dunno.

I also like the idea that she's some sort of enemy or the agent of an enemy, sent to insinuate herself with the Doctor with the help of a "complete guide to the Doctor's appearances in time" that has been compiled by various baddies (or perhaps one variety of baddie in particular) throughout the ages.

i liked river song and to be honest i would like to see her and the doctor to get closer i do not think that she is a baddie or at least i hope not.

I went back and watched The Empty Child, since there are so many references to those episodes, suggesting a review is in order. (The Poison Sky-Doctor wears mask and asks "Are you my mommy?", Doctor's Daughter-Doctor is given the same wound on the back of the hand and then becomes a father, Fires of Pompeii-"It's Volcano Day" this conversation happens in the 2nd part The Doctor Dances, The Digital blast pattern from the gun blast seen by Jack in The Doctor Dances, Fires of Pompeii-"Doctor Who, it's more than just a question" etc...). While re-watching The Empty Child I think I found the answer - Professor River Song is The Doctor's Mother.

Over the course of this season the Doctor has "become" the empty child at times. In The Empty Child The Doctor meets Doctor Constantine (aka the constant doctor). This scene is interesting because The Doctor is led by Doctor Constantine, who has the answers, while The Doctor has the questions. The Doctor also says "this is impossible" a phrase usually uttered by those around The Doctor while he gives the answers to the riddle. Before succumbing to his injuries Dr. Constantine says "Before this war began I was a father and a grandfather, now I'm neither, but I'm still a doctor." This sentence could have been said by The Doctor himself, in fact his response is "Yeah, I know the feeling."

In The Doctor Dances (the 2nd part of The Empty Child) Rose asks "...Always are you my mommy, like he doesn't know. Why doesn't he know?" A question that could also be about The Doctor, why doesn't he know who he really is? The only logical answer is that he doesn't know his parents.

If River Song is The Doctor's future wife why isn't she WITH him? Why does he only come when she calls? Since he's said countless times how he's rubbish at wedding, commitment, domesticity etc... it's odd that he would marry again. On the other hand, he would come when his Mother needed him, he would give his mother his sonic screwdriver. A mother would call her child "sweetie" as River Song calls The Doctor when she sees him. A mother would tenderly and lovingly caress her child's face. She can't be a Time Lord, because they always recognize each other, so she can't be The Master either. He is a time traveler, who better to give birth to a time traveler than an archaeologist?

When last we saw Dalek Kahn, the last remaining member of The Cult of Scaro, he did an emergency time shift to escape from The Doctor. It's possible he will want to destroy the Doctor by stopping him from ever being born. Without The Doctor to stop them The Dalek's would have been successful time and time again, they would have won the Time War and Davros would probably still be alive. I'm thinking that this might be where The Doctor finally meets his mother. It also makes sense that he would give her a diary to know which regeneration she has run into at a any given time and how much he knows at that point.

Jenxer you are amazing!!!! That is the best theory I've heard and you've sold me on it, great job!!!

she could've got the screwdriver from the doctor, he gave sarah jane a sonic lipstick.

the mother idea is a good argument, but i still think she must be a future companion.

i think there will be a problem with the time lines and her memories of the doctor will slowly fade away as the future changes.

i thinks the doctor will eventually open the little book but he wont find any spoilers because they wont have happened/ will be going to happen

wow that was a amazing theory!

Very nice theory, JenXer. Are we assuming, though, that the tenth Doctor (like the eighth before him) is half-human on his mother's side? If not, why doesn't he recognise her as a Time Lord?

OMG i know its only like 17 mins before dr who starts but ive just clicked...on the "next time" preview river songs says "i hate you sometimes" and the doctor says "i know" hmmm who hates the doctor but has a close relationship to him....thats another idea....jackie tyler hates the doctor XD lol well i was only joking there but oh well we will see in 15 mins :) cant wait

Now that its over what is your opinion of her??

i have no idea....she said his real name, he wont know unless he read her dairy. But im still dumfounded...i have no clue, anyone else

I was thinking she is just a future companion, but I don’t know that anymore because he said “You whispered my name, you could only know my name if…”, so I’m thinking she is someone but not just anyone!! Maybe his mother… I’m linking that idea more and more!!

Could she be Susan ? The very first Doctor's Grandaughter ? She said she had never seen him so young. . . and she might indeed know his name. Very off the wall thought.

As someone else mentioned, there have been so many stories about children this season; not only children and parents, but the many different ways different races use to propagate the species. Partners in Crime - the Adipose were illegally seeding a class 5 planet, Planet of the Ood - the Ood were being bread as a slave race by lobotomizing their psychic connection with the "parent brain", the Sontarans are a clone race, the survivors of the Messaline breed a new person from the cells and information gathered from a single donor's skin cells, in The Unicorn and the Wasp, Lady Eddison gave birth to the child of a Vespiform, gave him up for adoption and never saw him until he returned as a grown man...

It's possible The Doctor's mother didn't give birth to him in a traditional way or even know she was a genetic part of the child. It is also possible that the Time Lord father insisted on him being raised a Time Lord, and humans weren't allowed on Gallifrey at that time so the mother couldn't go with, or the time lord father died and another time lord took the child back to Gallifrey. (I've always gotten the feeling that he was an orphaned child.) Time Lords are psychic and Time Lords choose their own names. It's possible that a Time Lord infant will tell their mother their name in utero. Many ancient cultures use the idea of a name having power, so it's possible that a Time Lord's real name is always a closely guarded secret, which is why they choose their own name in The Acadamy.

Anyway ... I think the fact The Forest of the Dead gave more evidence that River is The Doctor's mother. River says that The Doctor turned up on her doorstep with a hair cut and a new suit. 1st - if they were married he would not show up on HER doorstep, he would come home, or at the least show up on THEIR doorstep. 2nd, he showed up with a haircut and a new suit, isn't this the stereotypical way every mother wants their son to come visit? Finally, in the dream life River is given in the computer, she has become ... A MOTHER. Donna became a wife and mother, but River only becomes a mother. I'm thinking this is a questions that wont be answered until perhaps season 5 in 2010, but I'm convinced she is him mommy.

Ok here goes :)

Think back to the end sequence of doomsday. Rose telling the doctor that there are five of them now... The doctors surprised look as he says to Rose "your not?" And Rose pauses for a second and shakes her head then tells the doctor that its her mother that is expecting a child...

What if Rose was expecting?

No we have the Ood, telling the doctor "I think your song will end soon" with the empathizes on "your song"

River Song Tyler maybe?

RTD has said that this series will end with the saddest ending yet.. We Know Rose is about to climb aboard the Tardis one more time... Will she bring "River Song Tyler" with her?

Her child... The Doctors Child... Will Rose die leaving the Doctor to look after River Song?

A companion yes but also a Daughter for 2009/10

Just a thought :)

Ok - There are a couple of things that need pointing out here; it seemed pretty clear that River Song is a future companion, but more than a normal companion. River also kept insisting that the Doctor could not regenerate - why would that be? The whole point of her taking on the task to save 4022 people, and dying in the process, was to protect the doctor because she knew he couldn't regenerate. What's with that point? Does she meet him during his last regen and doesn't want to jeopardize that? Classic timelord info, if I remember correctly, is that they have only 13 regenerations. The master was an outlaw because he didn't want it to end so he started stealing bodies, among other things.

Then at the end, River is saying goodnight to THREE children; the two who were the same as Donna's from before AND another girl who was NOT a duplicate of the other girl. That makes it very interesting, don't ya think?

She said he couldn't regenerate if he tried it. As is dead. No more life. End of the road. Full stop. Do not pass go and do not collect £200.

The point being is if he died there and then, then he would never meet River thus causing many things to implode.

The third child she was saying good night to, "the other girl" was CAL, the child living in the computer. She now has a real parent to take care of her instead of the imaginary father the computer made.

I just had an idea what if she was Martha? Remember River Song says she has been to the end of the universe and so has Martha!

Yet the Doctor had met her for the first time in The Library. Plus she had never seen him so young. And Martha would be dead in the 51st centuary.

true, maybe she is a regeneration of his daughter

Who is not a Time Lady and therefor can't regenerate.

How do we know? RTD always puts in a twist

I believe an Academy has been mentioned on several occasions.

she is definetly a future companion. no doubt about it. so she might be in the fifth season for doctor who if david tennant is staying on!!!

I don't think she can really be a companion because she obviously isn't traveling with him since she always waits for him to come to her, and what kind of companion would that be.

I was just watching the Girl in the Fireplace episode and it got me thinking. A lot of what Madame de Pompadore said kind of made me think of River Song. Talking about how she follows the slow path while the Doctor is free to jump in at any point in her life, like what he does with River Song although we just saw them meeting for the first time. River Song also knew the Doctor's real name, which Madame de Pompadore could possibly have know since when he read her mind she learned everything about him (although maybe he shut the door that led to his name). Anyways I was just thinking maybe it could be someone that the Doctor is in touch with like Pompadore and has read her mind? Has there been anyone else like this, I haven't seen many of the old Doctor Whos so I'm not sure and I don't remember anyone else from the new series.

also Steve moffat said in the comentary that it was more comlicated than just being the Doctors wife. just throwing that out there.

blind_moon and JenXer: WOW, you guys are awesome! Now that I read it, it seems obvious, but I'm not sure whether I would have figured that on my own!
Actually, you know what?
All the comments have been soo helpful!
Thanx, guys!