I wonder whether surplus A380s may eventually turn up in cargo configuration, or whether that is even possible.
The B747 was originally built as a freighter for USAF but lost the competition to the C5A Galaxy. The B747 went on to be a successful pax aircraft and didn't need any where as much re-engineering to convert to a freighter as I believe the A380 would require. Have heard it said a few times by people within the industry who one would expect to know that an A380 freighter is highly unlikely, largely due to conversion cost.