Potentially, the A380 could be an outstanding freighter. It has a huge interior volume*, very long range, and excellent payload. I do wonder about the double-deck, though. If the A380 was intended to be a pure freighter, what would be necessary to load and unload freight on both levels? After all, that upper deck floor is almost certainly part of the load bearing structure**. You can't just take it out. Perhaps a combo featuring the upper deck for passengers and the lower deck for freight would be the way to go.

*Some planes are more volume limited than weight limited for cargo operations. Back in the late 1970s, the Air Force added over 30 feet to the length of the C-141A to create the stretched B model. The A model was volume limited and they almost never were able to load it to the full payload capacity as a result. The B model stretch added inflight refueling, making the plane much more versatile at the same time.

**If that floor isn't part of the structure, what's going to happen to the people on the lower level in the event of a hard emergency landing? Having a few hundred people drop down on your head wouldn't be cool.