The terraforming of the floating islands is good. Personally I'd rather see more colour involved, than a stone block pallete. Splashing some kind of colour in there, makes it stand out more, I'd recommend using the new block types, diorite and granite, they can make the statement you want. Also be more generous on the ores, its a mineshaft may as well give players lots and lots of ores (dont forget lapis). The custom trees seem random, perhaps having a pile of smelted ore, or that kind of thing, might be a good replacement. Also
@SSunny suggested adding some sort of organic creations, for example roots coming out the bottom. This represents the island has been torn from the ground, yet nature is still gripping on to it which would be a pretty cool concept. The map has potential and if these changes are made I think it could be used, good luck and well done