The current structure of expansions is terrible. It goes kind of like this:
X.0 - Level > Normal/HC Dungeons > World Quests > Mythics and Raiding (render previous options near useless)
X.1 - Minor content
X.2 - Normally a new zone, maybe a dungeon and normally a raid, renders all previous gameplay ineffective unless updated making it overly boring.
My proposed expansion strucle:
X.0 - New level cap, Normal dungeons, World quests and max-level quests + seperate relevant expansion systems
X.1 - Heroic Dungeons (that are harder than today's Heroics), New raid at Normal/Heroic difficulty (again, with Heroic also being harder than the current "heroic")
X.2 - Mythic Dungeons, New Zone, New Raid at Normal/Heroic difficulty, Mythic difficulty for X.1 raid
X.3 - 3 pre-raid Dungeons @ Mythic difficulty, Mythic+ release, Mythic difficulty for X.2 raid
X.4 - Final raid, story elements tied together and finalized
Staggering the impact of a new raid tier IMO is the most important thing, and expansion long relevancy with dungeons similar to all expansions before Cataclysm.