I believe Sasuke is Madara's trump card. The reason I believe this is in relation to the Obito vs Minato fight. Minato applied a seal on Obito keeping him from ever being able to control the Juubi... ever! The Uchiha massacre happened, leaving only four Uchiha: Madara, Itachi, Obito, and Sasuke.
Obito was carrying out Madara's plan, one that involved Kurama as an important factor to completing his plan. However, when Minato applied the seal on Obito, Madara had to find another Uchiha to take control of Kurama. Itachi is not around anymore. Obito rebelled against Madara's plan. The only Uchiha left is Sasuke, which I am theorizing Sasuke is Madara's trump card.
Sasuke is in a prime position to take control of Kurama. I look forward to seeing if my theory happens.
/thread