If you did Thousand Needles, I'm going to assume you're Horde. As such, the undead zone in that level range is Hilsbrad, or you could also try the later parts of Stonetalon. Stonetalon is Ashenvale's sister zone.
I'd give it til 30-31 to move on to the next set of zones, including the racetrack quests down on the Shimmering Flats.
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