Then maybe set minimum levels like there are now, but make it so they all scale up to 100? That way a person could continue to play in whatever expansion they want. Because right now there's no way you'll finish all the zones in an expansion before having to move on.
For instance, why couldn't I continue to level up on Azeroth instead of being forced to jump off to TBC or WotLK after lvl 62 or so(that's the level where exp takes a nosedive)?