the current forests are somewhat boring, i.e. they never change without player interaction, all trees look the same (especially in conifer forest), etc. perhaps they could be made more interesting with a little help from tile decay.
when decay hits a forest tile, if the tile is more than X tiles away from any other tree but less than Y tiles away from any tree, a tree sprout would have some % chance of spawing there, possibly with quality average of the trees in a radius of x tiles and the soil q of the tile, or just q10 i guess. if there are no trees within Z tiles, the tile could have a % chance of changing to a different terrain type, either grass or mudflat or possibly swamp, with a greater % chance if that tile borders another tile of that type. there would probably need to be some kind of regional thing in effect so that all the tiles in the region would change to the same type.
perhaps these trees could be somehow differentiated from player planted trees, and grow more slowly, even on their native terrian. that would then allow for the trees to get to a death stage, where they would fall over, leaving a stump and a fallen log or logs. i don't think many players would like it if this happened to their trees though.
when decay hits a stump it could have a % chance of rotting the stump, causing it to no longer be useful for wood blocks, but perhaps allowing for the growth of a new edible fungus. maybe in this state another decay hit would have a chance to make it rot away.
another thing that might help is having the wild crops behave more like forageable herbs (as well as more foragable herbs, berries, etc). they would still only be found on the same respective terrains, of course, but i think having a giant field of wild onions or hops growing in the forest is pretty silly, while i find the idea of finding a few wild onions in a cluster from time to time much better. these could be effected by soil q as well, i suppose, or they could just all be q10 like they are in their current form.
hell, maybe a few different fern types in addition to the current one.
this isn't all completely thought out yet, but i think that having trees in different stages of growth, and some chance for the shifting of terrain types slowly over time would make the world a bit more interesting. maybe it's something that could be tried out on the test server at some point?