Will never happen.
They are replacing the Marine One (Presidential) helicopter fleet with new helos based on an Augusta/Westland design, but those helos will be assembled in the United States by Lockheed Martin.
Air Force One flies into a lot of airports that do not, and will not, have ground facilities/equipment capable of handling an A-380.
Congress WILL NEVER appropriate money for the Air Force to buy an Airbus to replace the President's airplane. There is a provision of United States law known as the Buy America Act. Though its requirements can be waived under certain circumstances, its unlikely the A-380 as a Presidential aircraft would ever qualify for a waiver.
For example, the United States no longer manufactures passenger railway cars, or subway/Metro/underground cars for that matter. All such railway cars are manufactured overseas, and shipped to a plant in the United states for assembly. This is the Buy America Act coming into play, even though the re is no U.S. manufacturer for such equipment anymore. So if the U.S. military were ever to buy a fleet of Airbuses, quite likely these would have to be assembled in the U.S.