Well, here's my theory. Watching Wrathion's little ingame 'movies' during the legendary quest phases it obvious that the Burning Legion is on the way. I reckon 90-95 will be a full on invasion of Azeroth. Horde and Alliance eventually fight them off. 95-100 Alliance and Horde invade the Burning Legions homeworld for some payback. Obviously Demon Hunters will be the next class to tie in with the theme of the expansions.
But - here's my twist. You have to sacrifice one of your existing characters to Demon Hunter training aka the first class swap.
Blizz could put the onus on the players then to make the hard call on what alt or main they sacrifice.
Lastly if I was Blizz and wanted maximum $$$$ and player commitment, I would bring back the commiting to a year contract. I'd have some random Game available and the ability to level Demon Hunters for 1 month before the rest of the playerbase are allowed access when the new expansion goes live. You start at whatever level you reach.