Blizzard tends to test out new features in one xpac before going full throttle with it in the next one.
Examples:
Dungeon finder came (plus raid browser) first in patch 3.3 (mid Wrath)
this led to >
LFR implemented in patch 4.3 (replacing raid browser)
Sunsong Ranch added in 5.0.1 as a small optional progression path where you start with a plot of dirt and grow it into a garden.
this led to >
Garrisons in 6.0 with a mandatory progression path where you start with a plot of snow and grow it into a base.
So when looking for FUTURE features in WoW, let's think about CURRENT features that are either limited in scope and/or optional, and imagine them becoming mandatory/expanded in the future.
1. Followers
Currently, it is OPTIONAL to hire a follower in your garrison. This follower will help you in world combat on Draenor only. This is a perfect example of something Blizzard is likely to expand upon and make mandatory in the next expansion. I could see them having some sort of "super followers" in the next xpac:
1. Gear out your follower in all item slots.
2. Select talents and skills for them
3. Maybe 30 levels instead of just 3.
4. Possibly they join you in 5 mans and LFR (but not raids)
They become an essential part of your character. You are gimped without them.
2. More alternate universes
Currently, WoD stands as an introduction to the concept of alternate universes. Its only one world and one possible alternate timeline. Again, this is something Blizzard tends to like to expand upon. Also know that when Burning Crusade was originally released, the plan was to have portals open up from Outland to different worlds, and each world would be the basis of a new expansion. Alleria for example had gone to one of these worlds. This plotline was scrapped at some point, but I suspect its back and we might end up following Alleria (wherever she went).
3. World Puzzles
WoD introduced some interesting but OPTIONAL world puzzles involving lack of flight, such as jumping onto ropes, trees and walls to reach items, particularly in Arak. This is another thing they might take to 11. The next xpac might make these mandatory in some capacity, and might even add these to raids as an extra level of difficulty.
4. Eliminating the concept of bags / inventory
Blizzard has been heading this way for a while. More and more items are getting moved from bag space to just a list of items, with those you haven't "collected" greyed out as unusable. Memory is expensive and its a lot easier to just mark down whether or the item has been collected by a player rather than having the player move it around as a physical thing. The price would be customization and randomness. But it might happen. Maybe you no longer even loot items at all. All of the gear in the world can get listed on your char sheet IF you find it. When you find it, it pops up as available. Then you just click it to activate the item and suddenly you are "wearing" it. Quest items are a thing of the past. Its all counters now.
Those are my thoughts...