How could Blizzard make old zones relevant again?
Should The new World Quest System be developed for the old zones as well?
It would certainly give the feeling of a living breathing world back again, especially when paired with VOLUNTARY level scaling.
There could be a button to press at the end of the level bar which scales you according to zone specific max level, so you could complete new world quests in that particular zone, after you have completed the main quests for it.
(Maybe this would even eliminate the need for pristine servers, because world bosses in these old areas would help build the community.)