Just thinking with people never experiencing old content like my favorite Kara, why not change the player instead of the instance?
What if you could queue for Hyjal at 85 and each class be given scaled down base lvl gear and spells for the instance? A ratio where it would equate to what was available at that time and level. I realize there would be some flaws and making some things a generic baseline. But you wouldn't have to change much in these except have the drops changed to things like tokens and maybe mats that could be used after the instance.
It would basically re-open every instance and raid from Rage Fire on up. You could even ad legendary items that need to be pieced together from multiple ones.
I'm sure the conversion program would be extensive, but how extensive is creating an expansion? I think it would keep previous content alive. How many people have never seen Gruul? Or had to do BT in lvl 70 gear? SSC and MC back to life. Just a thought.