The very fact Madara used the Kyuubi to attack Konohagakure has to be the greatest mistake he has or ever will do (other than wanting to have the Power of God for world domination. Also, it's always a mistake for the antagonist to reveal their master plan to other people because a smart one would have them keep guessing. It's like they are asking their enemies to stop them) is because of this simple fact: When Madara used mind control, the sharing of minds allowed the pooch to see what was going on in Madara's mind. Hence, the Kyuubi knows the truth of the Uchiha Massacre and likely knows has info on stopping Madara's master plan.
The First Hogake's Fuuin no Sho (Scroll of Seals) that Naruto used to learn Kage Bunshin no Jutsu is very overlooked. Since it has many kinjutsu, it likely contains techniques that could stop Madara.
I wonder if Madara is even aware of the Kyuubi/Sage Mode Naruto exhibited when Nagato tried to control him.
My second favorite is this: Madara should read the fine print on the tablet about the Uchiha and the tale of the Rikudou Sennin. While the Rinnegan can indeed fully decipher the text, it also likely has a negative effect: While the tablet tells how to release the Juubi, it likely also tells how to stop it.
My absolute favorite of this section has got to be this: Madara's Tobi persona will be Madara's true persona. That is, when Naruto unlocks his full potential (I think Nagato's right Rinnegan eye could be transferred to Naruto somehow or Naruto will unlock the Rinnegan similarly to how Nagato unlocked it), it's obvious he will never truly comprehend how strong Naruto will become to kill him(hopefully, Sasuke also since it seems the only way Naruto can save him is by killing him).