Awhile back, I wrote a library[1] to wrap a hex grid around a sphere (technically, there are 12 pentagons so it isn't a true hex grid, but close enough) and perturbed the elevation via perlin noise and then wrote some cellular automata-type vegetation simulations.

Several types of plants would grow, and randomly catch on fire and burn half the planet to the ground, and so on. I had a primitive water cycle, too; water flows to low points or gets absorbed by plants or evaporates. When plants burn, they release their moisture into the air, which rains back down on the ground. It's interesting how small changes to the cellular automata logic can cause major changes in emergent behavior.

That was a very fun experience and I could imagine it being the basis of a game. And being a finite world (unlike Minecraft's practically infinite expanse) it's computationally tractable to simulate a complete ecosystem that's constantly changing; even the parts you haven't explored yet.

[1] https://wiki.haskell.org/IcoGrid