An Housing system. Player's own house! durrr...But still I hope so..
Cross-realm related thing is plausible.. Maybe something like cross-realm heirloom or bank.
Would be amusing if this was some clever way to find out what features people want by reading the guesses/hype xD
My guess is the "mentor system" which someone mentioned earlier in the thread (letting players temporarily reduce their level to play with lower level friends). It's in line with the previous features (letting people play together in more ways) and they've been using the gear scaling mechanic more and more this expansion (Challenge Modes, Item Upgrades, even the PTR testing buff). I vaguely remember them mentioning in the past that it's something they'd like to add.
Hopefully it's not simply the thunderforged items that was just announced.......having random chance of gears being higher ilvl, and giving 25 man raids more chance of having it drop.
Assuming, that Wow is turning into World of Daliygankcraft, I can expect something like Looking for Daily feature, that will allow you to aftomatically find a group for doing dailies toghether. May be even crossrealm group.
Me to WoW chat : "This ice epidemic is out of control. 3 horses have just tested positive in Victoria. Where do horses find Ice dealers anyway?"
Random king of the internet : "Probably from a drug mule."