Based on the fact that Davros' goal was to destroy the universe (silence falling) in the S4 finale, plus the person saying, "Silence will fall!" sounds like him.

Last time we saw him was on the exploding Crucible, but he was standing (erm, sitting) right next to Dalek "I has emergency temporal shift!" Caan, and we already know that Caan can violate temporal locks of the kind that protect the TARDIS (from when Caan rescued Davros from the Time War). In addition, in order to even prosecute the Time War in the first place, the Daleks need pretty good time travel tech, and they certainly have anti-TARDIS weapons. If Davros could get his hands on some of those, he might elect to play a long game of destroying the Doctor and TARDIS over a long period of time. This also sets up a pleasing potential three-way conflict between the Doctor, Davros and the Apple iDaleks.

The Black Guardian is a reasonably good guess, but the Black is always balanced by the White, and there has been no evidence of the White Guardian being involved so far. The Valeyard is a more likely foe to return (and indeed is set up by the Dream Lord, to a certain extent).