I voted for the areas beyond Faerun: Kara Tur, Maztica, and Zakhara.
Al Qadim had a very unique feel to it. I also would like to see Kara Tur return, as Rokugan seems to rely exclusive on Japanese sources. Maztica, I never was familiar with, although I have some knowledge of Mayan, Aztec, and Incan cultures.
All told, I would rather see the less covered areas explored. Places like the Moonshaes, Evermeet, Nimbral, and Lantan. The last two places are just covered with paragraphs in the FRCS.