The idea around World of Warcraft is that the lore of the original Warcraft is to be developed further and catered to the players to unravel as they go along. There is no linear progression planned out before WoW was released, that is forever constant in the mind of the developers. We have dealt with the Burning Legion once, in Outland. we have defeated The Lich King in Northrend, we have annihilated Deathwing and now most recently captured Garrosh and stripped him of his title. Everything changes constantly and the evolution of Warcraft and it's story is not set in stone. What i think will happen now, as we go further, is that we'll meet Queen Azshara and meddle with the trolls of Zandalari, as well as touch on some further Titan-related lore, before something occurs towards the end of "The Dark Below" (if that is the name) and something serious happens in terms of the Legion, which means the next expansion after the one announced in a few weeks, will be Burning Legion based, and Velens' vision of Anduin and the factions will be fulfilled.