Here is how I would like it to play out :
1) The Trolls are in open rebellion against Garrosh.
2) Huge Alliance victory against the Horde. Ex.: Domination Points gets wiped out and Garrosh's troops are forced to retreat from Pandaria. The game story leaves Pandaria for Kalimdor and Eastern Kingdom.
3) After the disaster of the campaign in Pandaria, some orcish officers begin to talk more openly against Garrosh. There is dissention in Orgrimmar.
4) Most Reliquary representatives were left behind. That was the last straw. Lor'themar withdraws from the Horde.
5) The Alliance begins to push the Horde appart :
a) Trolls : The Hero (you) is sent to provide support to Vol'jin against some kor'kron troops.
b) Tauren : Tyrande learns through her informants in the Cenarion Circle (maybe Malfurion himself) that the Tauren suffered evily from a draught in Mulgore, but that Garrosh sent no help, saying that supplies must be reserved for the war effort. The Hero is sent to provide some supplies through the Circle. Tauren loyalty towards Garrosh withers.
c) Forsaken : The SI:7 sends the Hero in a disinformation mission in Tirisfal Glades : "evidences" will be planted suggesting that Garrosh is conspiring to remove Sylvanas. The evidence will point towards the Kor'kron and some high ranking Forsaken. Sylvanas gets rid of the Kor'kron and executes the Forsaken "conspirators".
d) Goblin : The Hero is sent to bribe Gallywix. Or the Alliance bribes some other Goblin to betray the Horde. That Goblin's actions cripple Orgrimmar's war machine.
7) Now that Orgrimmar stands alone and is divided upon itself, the Alliance launches an attack on Orgrimmar.
Meanwhile, on the Horde side, the different rebellious factions will attempt to unite and will also attack Orgrimmar.
Or the Alliance and the Horde Rebels could unite for that operation.