Although Mackensen was a spineless monarchist -and thus might have pleased Wilhelm II, who knew him well, as a pliable successor to the courtman Moltke, he was corps commander (XVII. AK) at that time and hardly had the reputation necessary. After all, the land arms race was in its red hot phase and an all out war was perceived to be lurking ahead. In this phase, the military establishment would have pressed for competent replacement.
Therefore, in 1912/13, most probably one of the Chief Quartermasters would have succeeded Moltke - either Konstantin Schmidt von Knobelsdorf, a former military educator of the Crown Prince, or Hermann von Kuhl, or Georg Graf von Waldersee, who had good political connections. Another possible choice might have been the Quartermaster General, Hermann von Stein.
My bet would be Schmidt von Knobelsdorf, who had Wilhelm's trust - and was in a suitable position.