I'm not sure that the city maps work to integrate them into one large zone... doing a look at the maps of Ul'Dah, the Chamber of Rule would overlap over the top of the Ruby Road Exchange where there's currently sky overhead.

Ultimately this would be a lot of work for the developers to merge the two zones and make them work as they ought to but little to no benefit. Having the original cities as singular zones won't attract more players to justify the man hours needed to make it happen.