Sylvanas is the Mary Sue villain for Blizzard.
She's sexy, big boobs, big booty and has no moral compass. We do not really know her ulterior motives other than wanting to be leader of the forsaken.
she does not have an awesome "end game" vision planned. It is just destruction of everything that's alive.
Lets say horde won everything. Alliance is wiped out. Sylvanas will then want to wipe out all the living horde next.
That's just bad writing, man.