Option 1
My guess is that ALL dungeons and raids are going to be level scaled.
That means for raids:
lvl. 77-80 enters the room, boss gets HP such as marrowgar in ICC10 had.
lvl. 90 enters the room, boss gets HP such as raid tier boss.
Etc.
All dungeons allrdy have a standardized setup (all 85 heroics, bosses had 504k health and did equal dmg, just different skills for example. ZG dungeon: all bosses had 1.1 mil health, etc.) So they can easily scale those aswell. All dungeons come with new achievments, level appropriate loot, etc and challenge modes ofcourse.
Option 2
They are introducing some filler content before the next expansion: Rebuilding Azeroth. More dailies, minigames and stuff, to rebuild the land that deathwing tore appart. During the digging we find Nzoth and the nerubians, wich will be the teaser for the next expansion.
Option 3
Crossrealm everything. Dungeons, grouping, guilds, raids but not limited to the current realms: all the realms.
Option 4
Minigame goldreward system. WoWs version of games such as bejeweled wich give ammounts of gold when u reach certain scores. Casino games, stuff like that. By whom? Gallywix ofcourse!
You heard it here first.
Option 5
Mirrage raceway is rebuild and players can now join in on races to become the next mirrage raceway champion. Buy a cart, tweak it with crafted material, gold, personalized stuff, and take your friends on in a Mario Cart like gamestyle!