River/Melody has regenerated twice, which is not the same as dying. River dies in the library in The Forest of the Dead. There is nothing to stop River meeting the 12th Doctor because the timelines aren't chronological. There were various indicators in Silence in the Library that she encountered several Doctors, and I'd love to see Capaldi and Kingston in the odd episode together.
Problem is Moffat messed up River's narrative big time and ultimately made a real hash of it. Instead of letting the mystery of River play out over a number of series he rushed at it and it didn't work out well in the end. But there is, I feel, some unfinished business there so maybe she will be back, but I can't see her being killed by the Mistress - that would be another situation where Moffat thinks he can muck about with the narrative because he 'owns' it and doesn't think other people are smart enough to work it out. The are so many loose ends and dropped plot lines with River that I think Moffat lost control of what he was doing in the scripts and forgot to keep track of the foundations he laid down in SITL and TFOTD.
Remember that she stated her Doctor was an 'older' one and that she travelled with him for a bit? I think her Doctor is Capaldi. It was a strategic casting from Moffat to run the River storyline into the ground further, I think. Yeah it's been stated on several occasions that River met several Doctors. Regardless of whether or not River died, I still would love to see her come face to face with Gomez's incarnation of the Master. River, in my opinion, also has psychopathic tendencies, and as this female Master doesn't try to hide the fact that she is completely off her nut, the dynamic between the two characters would be a sight to behold.