MS abilities usually complement each other i.e : Itachi has strongest genjutsu and strongest ninjutsu, Sasuke has Amaterasu and Enton, Obito has Kamui and Kamui snipe
This means that Madara would have to have two abilities that complement each other like Itachi's dojutsu or are closely related like Obito and Sasuke's dojutsu.
My theory is that his MS feats granted him the power to match Ashura's feats :
Left eye : see through shadow/mokuton clones
Right eye : use Susano'o clones
3rd power : Perfect Susano'o