Trees grow bigger over time, depending on what kind of resources are around them.
If conditions are right (water, light, air, temperature), spawn a leaf block connected to other leaf blocks or logs that are ultimately connected to the ground.
Spawn a root system replacing dirt blocks or moving them out of the way.
If there are enough leaf blocks in an area, spawn a log branch at the bottom of the group, connected to the trunk.
If there is not enough light, water, dirt, whatever, de-spawn leaves, make floating branches fall to the ground (not drop as an item, move down to the ground), and replace root blocks with dirt.
(I’m using language rather loosely in this to explain what I want to happen. Keep in mind that branches, trunks and roots are all just wooden log blocks, defined as different things based on what other blocks are around them.)
It would be cool for another mod to extend this for generating ores. For example, you could have an Iron Tree that produces some kind of iron fruit, or grow iron wood or something, as long as the root system is connected to a vein of iron ore.