Szilla over at World of Raids has a full report on the new Dustwallow Marsh changes coming in 2.3.

The zone is turning into a new quest hub for level 30-40, a painful gap in leveling. A new goblin town has been added in the south and various quest NPCs have been scattered around the map, ranging from the zeppelin crash site to Tabetha's shack. For those experienced in Dustwallow Marsh questing, the Shady Rest Inn questline receives some closure and there may be some more information about the missing kind of Stormwind.

Alongside this, we can probably expect more low-mid level content as time goes on according to Bornakk:

We would like to revisit and implement new content for Kalimdor and Eastern Kingdoms, but we have no estimate on updating all of the zones that people have talked about or if this will happen at all.

We always have to consider development time versus what is the most fun and currently we feel it will be more exciting to create new continents like Outland and Northrend, but we never know exactly what the future holds.