What if all the old world maps, vanilla to shadowlands had rotating and event-timed WQ's with a "Time Rift Portal" icon on the map when event was running, when you went there you could step into a portal to a "alternative" version bubble of that area and do the quests that are already there but they would be slightly altered by "time" in different ways and give current rewards.
This could be a cost effective way to reuse previous expansions and tie the world together.
Could also fit theme of current expansion.
Would work something like a combination of the invasions from legion, timewalking and the time questline from df.