Quite a key quote from the trailer is Jaime Lannister telling Joffrey that the war isn't over. While the Lannisters may have won their war against the Starks, they have plenty of other enemies inside and out. In fact Joffrey's assumption that he is beloved by all the winner is liable to come back at him pretty much immediately. I say this because a whole bunch of things that we saw in the trailer is due to happen following the death of Joffrey. Naturally this creates a power vacuum at the head of House Baratheon and the King of Westeros. This vacuum will be filled by Joffrey's younger brother Tommen being placed on the throne. Of course, he's very much a child and so he is little more than a pawn of Tywin Lannister's scheme. Furthermore, in order to strengthen their grip on the Lannnisters, the Tyrells arrange for him to marry Joffrey's widow Margaery. But others seek to make Tommen their pawn, including his mother, Cersei. She seeks to act as Queen Regent in his name, and of course the various members of the Small Council want to control Tommen and use him for their own gains. Ultimately Joffrey forgot the key lesson of the game of thrones - you either win or you die.