I think it would be nice if forests and even plains slowly repopulated themselves with trees.
How I imagine it working is that when say a forest has bellow a certain number of trees per tile the game would spawn new trees within that forest. It could place them a number of ways but mostly I think it should be weighted but random. I like the idea of having the trees spread, by this I mean that say there was only a small group of trees in the forest left that the new trees would spawn around the edge of this group and over time if left alone it would grow to fill the boarders of the forest. When I say weighted you'd want to avoid well walked areas and other trees and building etc.
When it comes to the plains things would most likely work differently but again new trees should appear naturally in my opinion. It would be more realistic than the uniform tree arrangements I can imagine us hearthlings building.