Air blocks. :D
I like the soul sand bit. Perhaps replace the stone with colored clay instead to add some color. I don't know about the tree logs hidden in them either.
As for the trees, it looks like you used quartz as the trunk-- I saw FroggCrabs use mushroom blocks (not the actual mushroom, but the larger one) in a build of his and I thought that it looked like a new tree texture. I think it would be interesting to see those used instead of the quartz.
Also, your trees don't look "out of this world", just a tad random with dirt "leaves". Take it up a notch and give them character. I found these "piriballs" through an "alien tree" search and I think you could use that sort of ball shape bit as inspiration. Probably use clay blocks instead of dirt.
Terrain wise. You can probably add larger islands on the bottom area of the map but still keep certain areas open to the void.