Rani is the most obvious choice to being River and Moffat said it wouldn't be any of the obvious choices. That should rule out Rani & even a sane Master.
Other theories I've read online are:
1: The Tardis itself. An evolved Tardis that has been the Doctors companion all during his travels. The Tardis is a living being and would know all the Doctors secrets.
2: The Doctor's Daughter. Or more of a half clone really. Makes the flirting weird though.
3: Lucy Saxon. The master's wife. She killed the best man she had ever known and as his companion, he probably taught her how to fly the Tardis.
4: The other half Doctor left with Rose. The Doctor said he would grow old and die, but what if he did regenerate and after many years find a way back into the Doctor's universe. The original Doctor didn't like him too much because he was more blood thirsty than he should be.
5: Jack's, or the Face of Boe's daughter. In the early station episode, in the background is announcement that the Face of Boe had a daughter.
That's just a few of the theories out there. Moffat said we would find out next season, so unless they plan of regenerating Smith out of the series already, or ending the series, I don't see how the Doctor could be killed by River. But then again, Timey-Wimey stuff....