Some decades ago I read an article that seemed technically respectable (can't remember where, possibly Scientific American) which said that liquid hydrogen fueled aircraft would start to become economical around 1.4 million pounds. There was a tradeoff between fuel weight savings and fuel tank weight and bulk/drag (cylinder on top of fuselage). Lower fuel consumption per unit of payload might compensate for the greater greenhouse effect of water vapour and clouds (I'm not up to the data and math). However if the A380 is too big then immediate prospects for hydrogen fueled aircraft don't look good. In any case it would only be feasible for very high-traffic routes.