I'm not quite as sure as you that 5.4 will be something other than the Siege of Orgrimmar. If Blizzard has nailed down a faster patch cycle, it is possible that they are trying to meet their goal of 18 month expansions. If that is the case, I simply don't see enough time for them to squeeze in a 5.5 and 5.6, as 5.2 would be March, 5.3 would be May, and 5.4 would be September, meaning 6.0 would be in March or April 2014.. However, if they want to keep the 22-24 month cycle for expansions, then it's possible that 5.4 could house a raid about the "Darkness" under Pandaria.
The main problem I see with dealing with that "darkness" in a raid is that we already know which Old God it would be: Y'Shaarj. And, according to Blizzard, Y'Shaarj is "very, very, very dead".