I don't see Ferrari employing two Germans. And I don't see Rosberg leaving Mercedes anyway, because there is no better place for him to be. It may be hard to beat Hamilton, but it is going to be harder in other teams and in inferior cars.

Only possibility is if Mercedes wants to take on a "challenge" and hire either Ricciardo or Verstappen (or Alonso) to see if either can take on Hamilton. Though top teams often like "stability", so from Merc' point of view there may not be much of a reason to go for another driver as long as they keep winning.