What are Madara Uchiha's weaknesses? At his current state, how could Madara possibly be defeated?

Any physical attack/Taijutsu would be easily deflected by either:  


The Susano'o  
The Gunbai (war fan, which proved to easily negate a Bijuudama)  
Shinra Tensei (which wasn't seen yet, but we have to assume he's capable of it).



   And I'm not even starting to talk about the Rikudo mode.


Any non-physical attack would be easily absorbed by the Preta Rinnegan path. There aren't many ultra-powerful genjutsu ninja who can hope to trap him in a genjutsu for more than 2 seconds. And in the offshoot that something does hurt him, he'd just regenerate. He's a zombie.



What weakness does Madara have that could be exploited to harm him? They'd have to damage him badly to hope for a seal.
It does not seem like any of Madara's new abilities have a counter to sound genjutsu, especially when boosted by Senjutsu / Sage mode.  While sound genjutsu and Senjutsu are both very rare, the manga has shown two examples of different techniques of that type being effective against techniques that Madara has recently gained.

The two Sage Toads' sound genjutsu is effective against the Rinnegan.  When Jiraiya had them use it in his battle with Pain, it was able to easily overcome 3 paths of Pain.


   When Kabuto in Snake Sage mode used Tayuya's flute sound genjutsu, he was able to immobilize both Sasuke and more notably, a Susanoo-wielding Edo Tensei Itachi.  The pair were only able to escape by using Sharingan genjutsu on each other.