Another Wild Theory From Your dad (oO Your dad Oo):
How is Itachi leaving Sasuke alive making Itachi gain more power? What is he gonna pull a Naroku and absorb Sasuke to make himself(Itachi) stronger? That doesn't make sense. If he only wants to fight stronger people then he is in a organization full of stronger shinobi. If he wants to fight a strong(er) MS user, or sharingan user in general, he could have left his clan alive. I bet someone in his village had some evil thoughts or intentions. If Itachi were to leak out information of Madara's power, some would attempt to make themselves stronger. Just leaving Sasuke alive to fight a strong MS user is a dumb reason for all the events that happened during and after he slaughtered his clan.
I still think Itachi knows Sasuke's potential. He wants Sasuke to surpass himself so that Sasuke can defeat Madara (if Madara is alive.) Itachi must already know that he, himself, isn't stronger enough to defeat Madara alone. Maybe the secret to unlocking the sharingans full ability is to know what it feels like to have everything precious taken from you. That's why Itachi killed his clan and left Sasuke alive. That's why Itachi told Sasuke for him to become stronger he would need to kill his best friend.
Also, if Tobi infact Obito. The maybe Obito is a direct decendant of Madara. Or maybe it will be like Yu Yu Hakusho where there is a demon gene that will awaken within him(Obito; if it hasn't already) which will allow Madara to be reborn within Obito. Maybe that is why it seems Tobi has authority over Pein and maybe he really meant Madara's power is within him.
Just Another Random Rant By oOYourdadOo a.k.a. Your dad.