Diablo 3’s next major update was mentioned briefly at Blizzcon, currently ongoing in Anaheim. Blizzard’s Battle.net though has all the details on what is coming in the update, which includes Greyhollow Island, a new Adventure Zone area in Act V.
Eternal Woods and Royal Quarters areas have been added to the main game map, though the latter won’t be open until a blockade is cleared in Leoric’s underground location. Stash space is being expanded, and more legendaries are being added.
Some Set Dungeons are also being added, which will only be accessed once you complete and equip a class set of items. Once you complete that set dungeon you will get unique cosmetic rewards. There are 24 total Set Dungeons being added, each with a unique cosmetic reward. Patch 2.4.0 will be releasing after Blizzcon, in either late November or early December.