So should 70-80 in Northrend, and 80-85 in Azeroth 2.0, and whatever MoP does too.
But wait, wouldn't that mean that going from level 1 to level MoP would take months? years? That's a very long grind!
That's why there is the second part of this idea, the part that I intentionally hold by in order to cause confused outrage, much like when you tell someone, "you're pretty... UGLY!" And then they punch you because that's stupid.
Rather than explain all the details, which I worked out and are excessively long and dull, let's go with this: You can level from 80-82 in Vash'jir or Mount Hyjal. You can level from 70-80 following a path starting in Howling Fjord or Borean Tundra and ending in Icecrown or Storm Peaks. There are multiple paths, of similar difficulty, time, and at least theoretically, fun (Vash'jir was a terrible, terrible place). My suggestion is to do the same, using Azeroth, Outland, and Northrend as the equal-but-different leveling paths. Behind th3e scenes there would be a few mechanical tweaks which would allow players to, if they wished, stop leveling at 60, 70, or 80. In effect, the level 'cap' is 60 and than all of those zones are tuned to give a proper amount of experience along the journey from 1-60.
Here's a diagram which might help. Or I wasted a lot of time.
Notice how if you don't want to do Desolace you can instead do Grizzly Hills. Or if Icecrown isn't really you favorite place, you can run BRD a thousand times.
I don't know how raids would fit into this. Or gear. Maybe the level one noob gear could become green-quality heirlooms that update every ten levels, so they are always pretty awful, but you won't be running around naked if you go from Grizzly Hills to Nagrand and find yourself unable to wear your gear, or alternatively, going from Nagrand to Grizzly Hills won't leave you ten levels behind on gear. In the world this might not be too big of a problem, but I think instance groups might complain.
The real point is that I think it would be a good idea to add a mechanic or some set of changes which allow players to jump to the expansion they want and level at a reasonable pace, completing quest chains and instances, without jumping off or falling under the leveling curve. And I made a chart!