Im going to guess (due to the game files being in the game for a long time) that 4.2.0 will probably be an Emerald Dream patch.
I dont think Blizzard will make an Emerald Dream expansion as there isnt enough Lore left that hasnt already been covered to justify making it into an expansion, however there is enough to turn it into a a major content patch.
The game files are fairly large and still in the game, blizzard will surely make use of them in some way, shape or form, even if it is just in the form of a raid + one or two pre raid dungeons.
Currently we dont really have a great idea of how much the Emerald Nightmare is taking hold of the Dream, or how well Yresa is dealing with the corruption, with speculation that N'Zoth is the one corrupting the dream mounting up this could be where we gain confirmation of his existence only to take him down later.
Im of the opinion (Just guessing though) that Blizzard will throw a spanner into the works and Deathwing may not be the final boss of this expac just as we have seen previously with TBC and Kael'Thas "It was merely a set back" Sunstrider" and Illidan "You are not prepared" Stormrage...
Remember Im talking about a PATCH, dealing with a small sub story that could logically be dealt with in Cataclysm.
Thoughts please