I would like to see trees drop saplings when they get to a very old age. And these saplings would plant themselves depending on wether they are in a viable location or not.
An example would be a spruce tree gets very old, And drops a random number of saplings, A sapling may slide down a hill into an area that has no nearby trees, And also is the Native biome for this type of tree. So the game would allow this sapling to plant itself. While another sapling that was ejected from the tree got hung up on a rock, And is too near other trees to allow it to plant itself. It would then lie on the ground as an object until either a player picks it up, Or it is deleted by the game.
This wouldn't work very well on flat ground, So maybe the trees would need to eject the sapling a short distance. Possibly simulate it being blown in the wind.
Another example would be that a player goes to the beach and brings home palm tree saplings, Which he then plants around his house.
However, He lives in a non sandy area. So the palm trees would never be able to replicate themselves due to improper terrain. But the player would be able to pick up the fallen saplings and plant them manually. Given that he is there when they hit the ground, And they are not deleted by the server first.
Trees should replicate according to their environment, Such as a spruce in grasslands can have a higher density of trees. Where as a spruce in frozen tundra would be a lot thinner. And a snow biome would have the least amount of trees.
This would also be a good idea for flowers and shrubbery. Although their distances apart would need to be very big.
On populated servers when you first spawn, There are no trees, Unless they are protected by the admins. Which is also completely useless to new players. A lot of servers line their roads with trees that are protected, And yet within the unclaimed lands there isnt a single tree. And you literally have to travel enormous distances just to find a few saplings to bring home.
If trees could self replicate, Then all of those trees that line the roads on these servers would start sprouting saplings into the unclaimed and claimed areas alike, And would be much more realistic than it is now.