I have a thing for trees myself, imagine a forest that grows ON A GIANT TREE. This is almost like Teldrassil in WoW, simply amazing.
But I’m not sure how this tree could be implemented, since I doubt it can be truly endlessly growing up – anything needs an end, which is fine, as long as it has a lot to explore. Additionally, a tree-world like that would mean that this world would be mostly vertically-oriented, which could be challenging (in a good or in a bad way) to move around and build on.
Great suggestion, though!