I would also like to see this and when animal AI becomes more advanced, animals should eat local vegetables but plants/trees/vegetables should be able to spawn saplings within a certain radius of the matured version. Animals should only eat matured plants and a sapling should possibly spawn nearby immediately after the parent matures and fruits. Finding vegetables may or may not be more difficult depending on how the spawn rates of vegetables and of animals is set (would have to be tweaked as time goes on).
The only issue I see is, as you said, they would replicate on their own which means they will start growing in unintended areas. But I suppose lawn maintenance is something that we would have to do in real life. In that regards, grass should also re-grow and spread.
Perhaps an addition to the API would be useful to set whether or not automatic re-growth is allowed in a chunk.