I think we're being lead by the nose where River's concerned. The impression we're being pretty-well forced to draw is that River is going to do something bad - something that will harm the Doctor or someone close to him.
Trouble is, her meetings with the Doctor are in reverse order. What we'll see now is really her introduction - the first time he learns who - and maby what - she is. If you step back and look at it from the Doctor's personal time line - which is effectively the same as ours - she can't be that bad because he is then - according to her, their diaries and other information - going to become very close to her. Unless she's been lying about them being close in his future, whatever she is or does, he likes her enought for her to later become a companion and/or lover.
I'd suggest that whatever she does that she feels guilty for - guilty enough to (mostly) sray in a prison tha she can walk out of anytime - is actually a sacrifice on her part - something that she hates having to do and regrets but was done for the right reason.
I do, however, think that there is something otherworldly about her. The reversed timelines and her memory remaining intact after Big Bang point towards her being something outside of our universe's laws.
From a practical point of view, she's a character that is supposedto exist in the Doctor's future - to know his future incarnations. For that to be possible - in terms of using the character in far-off episodes - the show's makers will need to be able to recast the part at somepoint. It's easy enough to have an alien species re-appear 50 years later - a Dalek or Davros is just an all-covering costume and makeup, for example, but the way they got around recasting "human" characters like the Doctor or Master was to have them regenerate. If River is going to be seen in - say - 10 years time - they may need to come up a similarly "alien" way of keeping the character whilst using a different actress.