Dalaran had its time. Silvermoon is another story, but when it comes down to the brass tacks, do we need to fly around this thing for any other reason than the novelty of it? If we're being honest with ourselves, we admit the answer is no. And that's a huge pile of work to commit for a novelty.
Now, portals between cities? Sure, but only between each city and Org/SW, and only if you take the time to visit the city once the old-fashioned way and activate the portal yourself, like a flight master. That'd be how I'd do it if it were me. It would remove the need to put a portal circle and MoP transit in every city, and help populate all the old cities in addition to the main ones. With cross-realm, it shouldn't empty Org or SW out too much.