Everywhere! I'm so curious about Thedas, and we've barely seen any of it in two games--just Ferelden and a small part of the Free Marches.
Places I'd definitely love to see: Orlais (especially Val Royeaux), Antiva (specifically Antiva City--Zevran's description of it makes it sound so interesting!), Tevinter (Minrathous!), Nevarra, and the Anderfels, definitely. I'd like to see Kal-Sharok too, and maybe Par Vollen. Loads of places!
Also, I think it would be fun to go back to Ferelden for a bit and see how things have changed since DAO--see how the Circle is doing in light of the mage/templar war, see Denerim and the royal in charge (though that would be complicated, since it can be Alistair, Anora, Alistair and Anora, or a Cousland and Alistair or Anora in power), and see Orzammar again. I'd really like to see the Dalish settlement in the Hinterlands, but like I said, complicated. Still, it would be nice.