With stuff like Battle-Field Barrens and Timeless Isle, it's quite clear that Blizzard are trying to introduce some dynamic content into WoW as well as a renewed sense of exploration. I think this will certainly continue into the next expansion. And I think that if we are indeed going up against Azshara next expansion, that the Great and South Seas will be the perfect next step after the Timeless Isle.
In an Azshara expansion it would be logical for much of the action to take place in island zones like Kul'tiras, Zandalar and the Broken Isles etc. however these zones would be spread across an area which may well be bigger than the rest of Azeroth put together. This would cause the same sort of problem we got with Cataclysm, with the world feeling disjointed. So I think it would be good to have a fully explorable ocean, navigable by aquatic and/or flying mounts, personal boats or Guild Ships. Split into two mega-zones, the South Seas and the Great Sea the Ocean would be filled with random events like pirate ships, krakens, storms, sunken wrecks etc.
Hidden Islands could also be a cool feature. Not proper zones in their own right, but just small little pieces of land with say a troll village, or Highborn Ruins, with their own discoverable little events and treasure chests. If Blizzard ensured they were suitably hidden and out of the way, these islands could still be discovered well into the patch cycle of the expansion. Discovering them on your own without looking things up on wowpedia would provide hours of content. Some islands could come with their own little scenarios even.
(P.S Before anyone says, yes this is similar to what they're planning in AC4, but hey it's not like AC has Krakens :P).