On a recent interview with Radio 1, Matt Smith gave some glimpses into series 6.
About the River Song story line Matt said "Whats amazing about Steven is, he's been plotting this since (Rivers) first episode. It's all connected. He's had this sort of 3 year plan and its all unfolding"
About the Silence he said " I think The Silence is going to be the scariest Who monster in a long time, definitely since the Angels, I think Steven's written a killer monster there"