One consideration with the progression system would be how all patches become consumable, right up to the end. Some players prefer particular portions, and once they are gone, they are indeed gone. Some will start late, even with a recreation of Classic.
Would Blizzard launch new servers every so often, to give those who missed out a chance to start at the beginning? Or for those who prefer a certain patch? Or for those who just like the term "F R E S H" (actually a buzzword)? Would Blizzard be willing to maintain servers at different patch progression states?
Then there is of course, The Burning Crusade! Which would be the next patch after 1.12. Blizzard would be opening a can of worms with full progression + multiple server states, as 2.x are the successors to 1.12. I believe Blizzard is scratching their heads over this too, which is at least partially why I'm thinking they will go cheap & simple.
But hey, I still like the idea of full progression
I don't want to demand it however, because it causes alot of hair loss for Blizzard staff.