There is a 5.5, it just isnt being called that.

[Matt0193]

5.5 was supposed to be follow on from Siege, with the Alliance making for Gilneas and the Horde rebuilding. It was also hinted at one point that you would find out who was the Warchief then, but that was changed to the ending for SoO. 6.0.1 will detail on changes that are happening, causing us as the Heroes to investigate, presumably at Karazahn (Khadgar being a key hero, having links to Karazahn, Malchaezar saying all realities and dimensions are open yo him and so on) where something's going off (Based on the speculation over the Karazahn string)and we're then directed to go to the Portal where the Iron Horde seems to be appearing. Cue Opening of the Dark Portal event 2.0 then 6.0.2/3 for the expansion release itself.

[sancha999]

In Warlords of Draenor, Garrosh escapes the pandaren trial and goes back in time, before the orcs first invaded Azeroth, to stop the orcs from drinking the blood of Mannoroth. His goal is to unite the orc clans under the banner of the Iron Horde. Using technology from his own time, he intends to march through an altered Dark Portal to attack this time's Horde and Alliance.