I have had this idea for a long time, but i thought it might not be very fitting to the game, but now since bamboo was announced, i think its the time for this.
Bamboo canes grow smoothly, their length and width increase over multiple levels until it reaches its highest point, but for trees, they grow from sapling to a fully grown tree in a sudden .
I think saplings should grow slowly, both their height and width should increase overtime, not in a sudden.
perhaps there should be 3 levels of width before it reaches its final fourth level, the full block size.
Level one width can be crafted into one planks block, level 2 gives 2, and so on.
thin tree trunks may also appear as branches on large fully grown trees and may be also usable to craft some unique blocks, like a wall made of tree branches stuck together.