Thinking that WoD will bring numbers back in the 9 million mark is living in a fantasy world. If you check previous expansion numbers, yes, the releases have always marked a jump in subs - but never a jump over 10%. I'd say it's mostly around the 7-8% mark.
We know we're currently at 6.8 mil. However, those numbers won't stay put for three whole months. WoW will mostly likely continue to bleed for the next 2 months. After which, with the new patch, and hype for WoD, subscribers will slowly come back, peaking at release. I'd say it's likely that in the next two month, the game will likely lose another 100-200k subs.
So we'll be sitting at 6.6-6.7 mil when the hype truly gets stuff moving. I'd expect a 10% bump from WoD, and that's being generous. So I'd say 7.3 mil in the two first months after WoD is released.
Trying to guesstimate how the game will fare during WoD's lifetime is just too uncertain to discuss, really. It depends on the content and decisions Blizzard will take. We know it will decline, though, and past the two first months, most players who came back will get tired of the game. So while I won't make any predictions far in time, I'd say the new year will see WoW dip under the 7 million mark again.