Thanks for the quick replies!
discbox: I'll run the Bonus Series'. Hopefully they'll give me a reason to go into that area.
Schanez: Thanks, but I already have Grandfather's Walk (or whatever that WB area is called). The area I'm missing is definitely in the Sundari Flatlands (my achievement log is adamant on this!).
You're probably missing the Dread Seed area, which is level 53 or above.. it's a part of "after Makeb" quest line.