Let's take a look at the moment when, at the end of SoO, Garrosh is finally defeated. The upper city is in chaos, Darkspear revolutionaries and Alliance forces are fighting with Kor'kron all over the place. The Underhold is conquered by the combined forces of both factions. Garrosh himself is shackled in his underground throne room, all major Alliance and Horde leaders are presented there, and the fate of Garrosh and the Horde are to be decided. At this moment, the Horde is really vulnerable - one precise blow could destroy it. The only reason why the Horde still stands is Varian's kindness - if it was not for him, but for Jaina to decide, there would be no Horde anymore.
Now i may understand Varian, but what about Sylvanas? This could be her "finest hour".
All of the Alliance and Horde leaders + Thrall are gathered in one room. Then, all of a sudden, the room becomes filled with Blight and all of them are instantly killed. Meanwhile, the Forsaken reanimate all the fresh corpses and take over the Underhold. Then, with all the chaos and confusion on the streets of Orgrimmar, they manage to take over the entire city. By slaughtering and reanimating everyone who was taking part in SoO, the Forsaken significantly bolster their ranks, which allows them to begin the conquest of Kalimdor.
Eventually, both the Alliance and remnants of the Horde will regroup and stand against the new "Lich Queen", but it will take some time. Horde forces are shattered and separated, and without leadership they won't be able to pose any serious threat for Sylvanas. Not on Kalimdor, at least. As for the Alliance, their reaction will be significantly slowed down by the sudden lack of leadership (which may lead to the serious infighting or even a civil war), as well as by the fact that the majority of their forces are located on other continent.
However, any Forsaken presence in the Eastern Kingdoms is doomed (combined forces of the Alliance and the Ashen Verdict will obliterate them), so Sylvanas would have to temporarily abandon that place. But if by doing so Sylvanas gets the entire Kalimdor for the Forsaken, then i bet she would be more than willing to pay the price.
What do you guys think of such an outcome? Is it possible that Sylvanas just missed a great opportunity for her people to become one of the greatest powers of Azeroth? I understand that such an event can't occur in WoW environment, but if we would be playing WC4 instead of WoW, i would certainly expect something like that to happen.