Basically, the next expansion is going to refocus around the original zones in Kalimdor and the eastern kingdoms, the main enemy is going to be the scourge and almost all of the vanilla raids are been remastered, such as zul gurub, molten core, blackwing lair and the dungeons are also been remade.
The game is going to scale back our levels and there is a massive stat squish coming to around WotLK numbers.
Kul Tiras will also play a part in this expansion as they look to expand their operations in Durotar and echo isles.
they are doing away with artifact weapons and going back to standard get weapons from raids etc.
all of tier 1 and 2 and the dungeon sets are getting an HD upgrade, alongside legendary weapons Like Rhok'delar, Longbow of the Ancient Keepers, Hand of sulfuras , Windfury ect, are all getting HD remakes.
Stratholme is going to be a major plot line and a raid will be released later on in the expansion where the alliance reclaims it as a major city, all major cities are getting a major revamp and orgrimmar is going to look like much like it did before cataclysm.
Naxxrammas and the scourge are going to be the main focus of the game and world quests will be scattered around the whole 2 continents mostly focusing on fighting off the scourge invasions that will take place across the map.
Jaina proudmore and thrall will play big parts in the storyline as will the new king of stormwind. looking to avenge the death of his father.
beleive what you will, that is the next expansion.