I'm pretty sure I read somewhere that Alex Kingston is shooting more episodes, so River should be back. Her story is not finished. We still need to see her final date with the Doctor before she goes to die at the library And given that Amy and Rory are her parents, I imagine she'd be involved in some way with their departure as well.
The series has never disappointed me in terms of casting or companions, and I'm sure Moffatt has some good ideas in place for the next season. (Although I do hope we won't get so much timey-wimey paradoxical solutions to the plot this time around. I still think season 6 was a convoluted and overly complicated mess that didn't live up to the potential of the premiere.) I do admit to not being super excited by yet another white female companion. I wanted the Doctor to have a male companion so we could have a proper bromance