I realize the tonnage issue, and that the A380F has two decks (three counting the cargo hold) that can only hold standardized pallets (the upper cabin floor in an integral fuselage structure that cannot be removed.) I've read Randy's Journal, too. ;^)
The A380F is optimized for max volume, while the 747-8F is better for outsized cargo. However, the 747-8F front load door is best only for outsized cargo. Most palletized cargo goes through the side doors, so the loading nose may not be put on a "KC-747." (The Air Force has lots of C-5s and C-17s for outsized cargo.)
In a Freighter/Tanker role, having an entire upper cabin for seating and the entire lower cabin (and cargo hold) for palletized freight isn't going to hit max gross weight before it hits max volume, and would allow an entire maintenance squadron deploy along with the air squadron, not just the flightline troops as is done with the KC-10 and soon to be KC-45.
There would only be a limited need for this size of an aircraft, thus it would only be purchased in KC-10 like numbers, i.e. 6 or 7 dozen vs. several hundred KC-45s.
All the 747-400 and 747-8 freighters have both a side cargo door and a nose cargo door. Some cargo operators might be interested in the 747-8F having a wider cargo door similar to the 777LRF side cargo door.