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.
Kil'jaedan isn't dead
We don't know how many Old Gods are still active
Azshara isn't dead and she still has Neptulon
"It's the content, stupid." Both in quantity, and in quality.