I think many people underestimate just how many older players who have quit because the game got too casual friendly, would actually resubscribe making a few TBC esque changes to the game.
If the developers removed LFR, and flying mounts from the game (i actually like the fatigue bar where you cant fly forever), also removed the heroic difficulty, so essentially normal difficulty is the hard difficulty, and flex became the new normal, millions of players would resubscribe.
With connected realms being implemented, the issue of low pop servers is essentially a thing of the past, so pugging for flex wouldn't be an issue for the LFR crowd. And bringing back a sense of accomplishment to the game, as well as reduce ilvl inflation through 4 difficulties would really go far in many people's books.
I know these changes would see me back in the World of Warcraft in a heart beat.
What changes would bring you back to the game?