Also, (major) things that Blizzcon can cover if WoD is released prior:
Legacy of the Void
Next Hearthstone solo raid
Heroes of The storm stuff - open beta/release dates, new features, etc
Diablo 3: 2.1 should be out by then, so I'm not sure. It's certainly not IMPOSSIBLE that they could already reveal the next xpac, given that it wouldn't be coming out for over a year, but being blizzard that is not likely.
Warcraft-specific:
Blood elf model reveal, if it hasn't been done yet
first major patch and post-launch raid
Anniversary stuff (10th for WoW, 20th for Warcraft) - wouldn't be surprised if that Warcraft 2 update for modern systems is announced as a free anniversary download..
Movie stuff (if they had a teaser ready for comicon, they will have stuff at blizzcon)
Patch 6.1 stuff, 6.2 teases. Everything about WoD was already well covered in last year's Blizzcon. What could they possibly talk about that would make for a wildly satisfying pre-launch experience?
I agree; even if WoD isn't out before Blizzcon, how will that really help blizzcon? There will not be any additional news or features to announce at that point.