If we didn't already know the name of the new treasure map dungeons ("Excitatron 3000", which still amuses me), I'd have assumed the "treasure islands of the Blindfrost" mention would be that.

The biggest mystery for everyone in the setting is probably Meracydia. The "Golden Cities of the New World" are mysteries to us as players, but they would probably be at least known to the people in Eorzea, due to the immigration of the Mamool Ja and the safe passage between Aldenard and the New World that Merlwyb found. Meanwhile, Meracydia still hates anyone from outside, and is said (in the lorebook) to still be mostly wasteland. I'd say an expansion in the New World would focus more on the "cool new place" exploration, while an expansion in Meracydia would focus more on rejuvenating the land.