Just a note: This thread is mainly just brainstorming ideas. Nothing I mention in any comment is something I feel SHOULD absolutely be implemented. Just some things that help create a more vibrant world that brings players back to the original fantasy that made WoW what it was in the first place.
WoW needs more evergreen content that would attract players back to old zones. Such as random zone events rather than one and done world quests. Those events could be dynamic and reward relevant items and gear based on your actual level.
If only WoW had went in another direction since Vanilla. Imagine if you will WoW expanded more inwards than outwards?
Let that sink in for a minute.
What if WoW utilized their phasing technology and built expansions that fleshed out the zones more, added new dungeons to existing zones, new enemies, NPC's etc, while still respecting the old ones? Help and NPC who is fighting off Kobolds from raiding his farm, he thanks you and that's that. Then after the expansion you get a letter from that NPC requesting your aid yet again. Those nasty Kobolds are at it again, this time they have tunneled under the farmers crops. You take care of business and are on your way. But wait there's more! What if during the quest you discover a tunnel deep within the makeshift tunnels the pesky Kobolds made. This tunnel leads deeper still to a forgotten grotto where magical barriers thought long forgotten still stand vigil over this long lost area of travel. You bring this to the attention of your local mage guild/wiseman, etc and it gets looked into.
Later down the line some expansions later, You receive word that those barriers are now able to be broken and a new party is forming to explore what lies beyond! Come join us Hero!
This type of content would not forget about the people you've helped before and make the world feel more rich and vibrant and alive. This is just one example. Imagine seeing actual progress in zones you helped, etc. If you join someone else on a different layer of advancement then you are simply given a warning box that explains this is a possible spoiler or some such, etc.
Right now WoW since Vanilla has made each new expansion a self contained game. This is part of what has slowed down development I believe. If only WoW had went into a different direction of building each expansion inwards instead of outwards then it doesn't mean that there could be no new zones, just that they would come slower and when it made sense to do so, while not abandoning the old ones.