I actually hadn't considered that but I would say that's a good argument for why 11.0 won't be solely a "backside of Azeroth" expansion. Because we JUST had that "wow cool new thing lets explore" theme, and unless the idea is to completely replace the factions with the Explorers Guild it would be too repetitive. (The item labeled "Frontier" is my one fear that they will straight up just do it again though... NEW FRONTIER!!!!!! GO EXPLORE FOR DA EXPEDITION!!!!)
I can see it both being backside and old world, but not just backside.
- - - Updated - - -
Except due to old-school leveling, you still spent a long time in the vanilla areas. And then Wrath directly continued many Vanilla plots that TBC didn't touch on, so they were connected in that way.