there is a market for very large cargo to be loaded without breaking it down but it certainly isn't big enough to allow development of an all new freighter - unless it's the military . But no-one is planning a successor to the C-17, C-5 or the An-124 so you're left with the odd A-400, the remains of the 747 fleet (maybe a few more modded to pure freight) and the possibility of a "Beluga" type mod to the A.380 or the B.777