A couple of notes:
1. To do the fancy stuff, like new grass mixes, you will need new grass skins and meshes. This gets tricky; basically, the requirement is that they're all on an atlas. I've done it and the guy who did the "whole world looks like fall" thing did so, also. The ancillary price is that you currently have to ship with a modified materials.brf or at the very least, a different grass texture file. This is problematic for a variety of reasons that I suspect only me and three other people care about, but be aware that it's not entirely risk-free.
2. You can do some fairly powerful stuff with this, by playing with frequency settings and adding new objects that can be spawned. For example, you can place a few village hovels on maps occasionally or make the grass a lot more dense (warning, this costs some GPU). Taleworlds was working on a "cluster" feature that would have allowed to do that and perhaps have clustered objects around it; the code's there, but I don't know if it works yet.
3. Anything that isn't fkf_grass gets drawn into the baked shadowmaps for the level's static geometry. This is usually a good thing, but can produce some strange results with stuff that's using the grass material that is not using fkf_grass, because the shader uses a distance-from-camera trick to hide geometry culling, so you can have shadows for invisible things. It's a tradeoff; ideally, you'd probably be better using a different material and incorporating a one-triangle LOD for distance LOD, but depending on how far away it is, and how large it is, you won't have perfect choices.
4. Trees are one of the most powerful examples in the file. Use the code to create various "forest mixes" of realistic terrain. I'm seriously thinking that I need to include stumps and fallen trees into my forest mixes, along with some rocks.
5. There's a fkf_rock setting. So far as I can tell, rocks are functionally identical to trees; it's a mesh and collision reference, they're both dealing with the ground in similar ways, and both can use LODs. The engine only rarely generates a "rocky" terrain, though, while usually generating considerably more trees and bushes. I think it's fair to say that in most cases, one wants to mix some rocks and other debris with trees to create a natural feel to forests.
6. It's doubtful that you can include more than one fkf_guaranteed item per terrain type. That seems to be reserved for grass.
7. None of the content generated can be searched, post-processed, or otherwise altered. Which is too bad- I'd like to randomize the density for grasses and things. But it's still pretty powerful.
8. Whatever code deals with which mix of objects is picked is black-box code, like the code dealing with AI weapon choices. I hope that as people play with this to create more interesting terrains that they figure out what does what and document it here.
Oh, and ... if anybody is bored... I'd like some desert plants and some sandstone rocks, to prototype a better desert with.
