Conway's game of life (a set of very simple rules that gives complex results) has been used in ADOM to simulate herb growth patterns. We'll want something more robust, naturally. Another example of growth patterns of plants are faerie rings. These are obvious, but there are notable differences between forests (large trees with empty trunks and mostly leaves on the bottom, or tightly squeezed and full of brambles etc.). It would give different places character. "Evil" areas would benefit from a thorny and tight vegetation, for example (cfr. the brambles of Mordor).
Well, I talked about this in one of the recently added sections of the Improved Farming Rebooted thread...
Basically, the idea is that you have a set number of sizes for plants - let's say 7 just because, although this has nothing to do with the 7/7 fluids thing. So, if a plant has 4 crowding, it occupies its entire tile. If the plant is a tree or something, it might have a 5, 6, or 7. If it's a 5, it blocks off anything but a 3 or less in the tile next to it, although diagonals might have up to a 5, as well. Hence, a 5-crowding tree could be planted as a checkerboard of trees with smaller 3-crowding plants. Then, a 6-crowder could have a 2-crowding plant nearby (which would be something fairly small that likes the shade, like a flower or vine) and a 3-crowder in the diagonals. A 7-crowder would force nothing more than a 1-crowder adjacent, and 2-crowder in the diagonals.
To start with, it's nice to have some variety. Finding out the exact relationship with cave formation is a later worry. (I suggest the worm Ourobouros and its kin, as a starting point.)
We already have ways to control the likelihood of cliff etc. in worldgen. This would go a bit further and allow bits of code (in the form of mathematical formulae, most likely) to be put in instead of just numerical variables into the same code.
I always thought of it as some sort of Zuggtomy-style giant mushroom demon that ate its way through the world's deeps.
I guess I just don't see what sort of sliders you'd actually get to play with in regards to how a hill forms, though. I mean, other than maybe something like gravity or plate tectonics. Unless you want a "make more hills" button, I don't see what you could do to really
change what a hill is. Or am I thinking you are asking for something different than what I'm thinking you're thinking. ("I think so, Brain, but if Jimmy cracked corn, and nobody cared, why does he keep doing it?")