Just to put it out there. I'm not particularly nostalgic. This is not an appeal to "bring back the good old stuff" or anything. I simply think Blizzard shouldn't throw their work away.
The pre-cata azeroth content isn't 100% outdated/incompatible.
Likewise, the world itself isn't 100% incompatible with flying. Some zones can be "copy-pasted" from post-cataclysm.
So, again, it's not "from scratch".
And the "running out of content" and "nowhere to go" was before XP was buffed.
You now easily outlevel a zone before you finish 50% of it.
Blizzard could simply add a way to visit pre-cata azeroth post level 80, possibly downleveled accordingly.
It's part of my "full" suggestion. I simply forgot to mention it.
Again, this system would stretch into the future, and could even become a way to allow Blizzard to reward you with achievements/loot that only makes sense while the content is "current", by requiring you to do it at the appropriate level.
Imagine Blizzard enabling you to acquire the Undying/Immortal achievement in Naxxramas by requiring you to be downleveled to 80 with iLevel cap.
It would make WoW feel like an evolving experience WHILE playing.
You'd go to Scarlet Monastery pre-cata and find the old set of dungeons/enemies, and then post-cata you'd find the new one.
Same thing for other dungeons.