Another possibilty could be that it is quite possible that Madara was alive at the time of Naruto's birth as he seemed to be aware of 'him' and therefore Itachi actually met the real Madara. Tobi killed Madara??
shisui needs zetsu parts as he had his arm taken from him
Anyway, lets see how it pans out , as I said above I don't see any one else being credible.
the other options are Uchiha Izuna and Uchiha Kagami and neither has any impact for me as their characters have not been shown (as of now)