Some quests was removed from game after Cataclysm, because Cataclysm destroyed some zones. So what about Creating Time Walking portal in Caverns of Time that will let you enter old zones for doing old quests before they was removed.
And since it will be instanced via TimeWalking/Portal in Caverns of Time like different Continent/Island , it will have zero effect on changing current lore,Legion Lore or Lore/Story of next expansions.
edit:
Instead of bringing back old zones they also can bring some old quests to new zones and mark them as Time Travel quests with blurr effect on screen or something, there is already few quests like that.
Or create few instanced dungeon-like zones similar to Old Hillsbrad Foothils and Old Stratholme in Caverns of Time.
Or simply backup few zones(as time travel zones) that have most interesting quests in case if they are going to be removed or destroyed in next expansions