Evergreen developed a 747 based water bomber about 10 years ago - before Evergreen went belly up. Global then picked up what Evergreen had developed and installed it into a 747-400 - the Global Supertanker. It has a capacity of ~18,000 gallons of water/fire retardant. Apparently it works OK - got used quite a bit during the big California wild fires last year. But, even with a ready supply of old 747s headed for the boneyard, there doesn't appear to be much interest in building another one even though the development work is pretty much done.
I don't know why the lack of interest in increasing the 747 water bomber fleet - perhaps a water bomber that big is just too limited and specialized. But whatever the reason, an A380 water bomber would run into the same issue(s), along with the added costs of development and the limited number of airports it could use.