Whatever happened to Kel'Thuzad's phylactery?
Also time is ripe for a full fledged Legion expansion.
Hm. Let's see what I can think of:
-Throne of the Tides (Neptulon's Kidnaping)
-Rift of Aln
-N'Zoth is still alive.
-C'thun has regenerated somewhat.
-Azshara is still alive.
-Thrall mentioned Gallywix's station is temporary.
-Wrathion has been warning of a Legion Invasion and Velen has seen an adult Anduin leading an army against them.
-What did Slyvannas do to Kolterra at the end of the Western Plaguelands questline?
-Why is Tirion sitting on his but as the Forsaken are sweeping across the Plaguelands. Who does he think deserves sovereignty of the land?
-What have the Knights of the Ebon Blade been doing since Arthas's death? 5.0 hints that they may have dissolved into Horde / Alliance lines.
-How can a tuskarr survive in Pandaria when before it was mentioned that pirates taking them away from Pandaria watched them die after like two or three weeks out of their climate?
-Are the blood elves still allowing the high elves access to the Sunwell after 5.1 (seen in Quel'Danar questline)? If not, will Quel'Danas be a scenario-type thing for the two factions?
-Bol'var, the New Lich King.
-The Skull of Gul'dan in the Sunken Ruins
-The Council of Black Harvest (they are JUST getting explored in 5.2).
-What's under Tiristfall?
-Why can't Yshaarj regenerate in the same manner as C'thun? Does his death hold the secret for forever stopping the Old Gods?
-What has Algalon been "observing" since Wrath of the Lich King?
Just a few off the top of my head.
Anything with the ethereals playing a main part.