The next expansion marks the end of all islands, continents and landmasses recorded in Warcraft's pre-WoW lore. With expansions showing no signs of slowing down, it is inevitable that Blizzard will soon begin adding entirely original continents for the players to explore; lest we resort to intergalactic spacefaring adventures and planet hop from expansion to expansion (and let's be honest here... really? C'mon...).
While we DO have a few remaining areas to -potentially- serve as expansion-sized material, namely the Dragon Isles north of Quel'Thalas and east of Howling Fjord, plus some unseen nations surrounding Eastern Kingdoms such as Crestfall and Balor, the areas would have to be served with a soft retcon to explain the lack of size and given interesting material to make them worthy areas to visit. Which isn't bad, it's just a repeat of what they did with Broken Isles.
So while we're no doubt a few years from seeing what the next expansion has to offer, or potentially even half a decade from another Azeroth based expansion, Blizzard has already settled on the next expansions location and have started concept art on it already (which they've already confirmed to be in concept stage of the next expansion after the next expansion).
Where do you think we'll see new continents land, and are you alright with sudden, out-of-the-blue entire civilizations cropping up into the lore? Personally, I don't mind, though many are still of the opinion that Azeroth is complete.