Itachi hinted that Madara isnt as powerful as he once was. Hes after Sasukes eyes..I think hes gonna succeed. I dont think Sasukes ready to take on Itachi. What I think is going to happen is: Itachi Takes Sasukes Eyes, Sasuke having the white snakes abilities may later grow a new pair(crazier things have happened in this manga), and Itachi will gain
the new Sharingan technique and become the primary threat of the series.
The Whole Madara thing seems like a back story. I doubt Madara will really be the prime threat. I think its going to be either Itachi or Pein.