I meant story wise. The events of Outland happen before the events of the DK starting area. There are Draenei there, so it is post-Exodar crashing at least and probably just after the Zombie event, since SW and Org appear to be zombie-free when the player arrives there.
The only thing with having a full leveling experience from level 1-58 or 68, is that you'd effectively be spending a ton of resources on creating an entirely phased or just completely different Continent, or several potentially, just for DKs and assuming that DHs get a similar treatment, DHs too. One of the reasons Blizzard felt that they dropped subs in Cataclysm was because they felt they devoted too many resources into remaking the world that it took away from other aspects of the game which resulted in a lesser experience. While this would've meant that WotLK would've had Cataclysm's treatment essentially, it's not really a feasible option.