In the quest The Red Crystal, you discover what looks as if it's part of a humanoid jawbone, encased in a red crystal, which has fallen down the mountainside from Felwood.  This could be part of the Skull of Gul'dan.  I was under the impression that after Illidan consumed the Skull's power he cast it aside.  I would guess that the appearance of the Skull (or a skull, anyway) in the Burning Crusade material wasn't created by the same people who created the Darkshore quest text, or that it was simply forgotten in the same way that the original story about how the Eredar corrupted Sargeras was forgotten.  Continuity gets sacrificed in the desire to make something new and cool.  I guess we're lucky that it happens as rarely as it does in WoW.

I would seriously doubt that there will be any further WarCraft strategy games until after WoW's last expansion has been released, whenever that will be.  Yes, perhaps the heroes of the Alliance and Horde will invade Northrend and destroy Arthas, and perhaps other fearful enemies will be defeated in future expansions as well, but there are plenty of loose ends that could be explored.  Hakkar the Soulflayer escaped from the Titans' prison; what else might escape?  Is Sargeras really dead?  Kil'Jaeden still leads the Burning Legion, and there is no content in which he is defeated (or even directly encountered) so far.  Harbinger Skyriss in the Arcatraz claims to be from an empire more numerous and destructive than the Burning Legion.  The world from which the Worgen came is not well known.  And those are just the things I can think of right now, based on loose ends -- since they made up everything that's currently in the WarCraftVerse, Blizzard can make up more things.