There`s a PTR quest that lends credence to 5.4 being Siege of Orgrimmar. "We come now to the climax of the pandaren campaign."
http://ptr.wowhead.com/quest=32598
That means 5.3 wraps up the Pandaren campaign. All the storylines on the continent have been wrapped up such as the Mantid and Thunder King. 5.3 will probably have Garrosh make another grab at the Sha, then 5.4 will be Siege of Orgrimmar.
At the end of the quest chain though even -with- the Divine Bell destroyed, he says he WILL find a way to master the Sha.