I've actually implemented one myself, they aren't too hard to do, not sure it is something generic enough to add to a MMO/RPG framework.
If you wanted something simple you can easily create a Buildingentity that can be placed by the user, and embedded in the prefab have several stages of growth in the form of children with different meshes, only one showing at a time.
Then just add a script that checks some time variable and then hide the current mesh and show the new one. You can even play some sort of leaf explosion particle system at the time of change to hide the switch and juice it up a bit
