It can be done scenaristically, just need to put the "old world" and its expansion in a time pocket, and decide that the "new world" is the new construct for stories, with more grounded problematics like ogres, dragonkin, bad Orcs etc. Pretty much the vanilla world.
There would nee to be a new capital for each faction, where the "portal" room would lead you only to the "old world", and future expansions.
Now, it seems the wow team is running on a budget to keep players hooked, so such an endeavour is unlikely.