Blizzard's big problem is they out-mode old content with their gear itemization and stat requirements. I haven't played since Cataclysm released, but effectively each new expansion contained content that made it pointless to continue working on previous endgame content, primarily because the gear itemization was so drastically changed. Obviously this is going to conflict with the episodic nature of the end-game content as Blizzard releases it. Even though they try to maintain a hub concept now to let them set level ranges in an attempt to avoid this, we still see things like end-game content constantly being dropped from players "do" lists.
Since itemization is the primary problem with the game leading to content being rendered pointless, I think it would in Blizzards best interest to finalize a level cap and no increase it any further, as the level cap increases are the direct cause of the itemization increases, and vice versa. It's a vicious cycle. If they finalize the level cap, they can revamp all the game content to make it viable, but in a less linear fashion.
They can keep the multiple zones of a requisite level range, but they need to make the gear from these ranges vary, not on stats, but on some other metric such as item style. That way it varies the content from zone to zone but it doesn't require someone to favor one over the other for itemization. Doing this, they could effectively return old content such as Molten Core to circulation. Imagine being able to collect Tier 2 armor with the same stats as Tier 8, or 9, or whatever they're up to now. I think a lot of players would be more than willing to play to do that. Not only would it add more variety to end-game, but it would add more variety to the player base with a huge number of viable styles to pick from.
If they want to keep releasing new content, they need to stop adding levels and increasing itemization requirements, and instead focus on plot, style, and creative boss mechanics to increase difficulty. The short and dirty of it is players need to have a tangible reward they can earn, through hard work.