I can't say I'm as concerned about this as I was a week or so ago-- the revelation that we'll finally see some new content in Dustwallow Marsh has me content (so to speak) about midlevel content for a while. But it would be interesting to know if Blizzard has any other plans to add on to old Azeroth (either by creating new zones like the Draenei and Blood Elf starting zones, or by developing existing zones like Mount Hyjal and Gilneas).
Bornakk, always the spoilsport, says what you would expect: while it would be cool, there are no plans right now. He also says, however, that Blizzard is "more excited" about working on new continents, like Outland or Northrend, at the moment. As usual, it comes down to the player base-- because Blizzard sees so many characters at 70, they're "more excited" about expanding the endgame than the midgame.
Bad? Good? We do know this: Blizzard has a history of paying attention to what their players do, not necessarily what they ask for. When Molten Core was packed with guilds every weekend, they made more 40 man raids. When Karazhan got super popular, they made Zul'Aman. And hopefully, when Dustwallow Marsh is overrun by players running alts through the middle of the game, Blizzard will finally see how badly players are scrambling for new content in the "old" game.