Boeing may eventually build a larger plane to replace the B777-300 and B747, but it will not be a "fattened-up" B747. It would be an all-new plane. Boeing has done serveral design studies. The one that seems most plausible to me is a large twin with a 270inch circular composite fuselage. It would have passenger seating only on the maindeck, though upstairs could probably accomodate crew rest seating and bunks, passenger sleeping bunks, galleys, and economy class lavatories. A double-decker is also conceivable, but I doubt it.