I think mapgen in the minetest engine is interesting, and there's a lot to play around with! There are several map generators built into the engine. Curious Garden uses the flat mapgen, with hills, lakes and caves included. This generates a map with large flat areas for building, some gentle hills for variety, and big and small lakes. Below ground, there are cave networks and huge caverns. You can alter some attributes of how these features are generated, to customize it for a particular game.A game can define geology, environments, how custom structures are placed, and run custom functions during mapgen for each section of the world. This is accomplished with a table of attributes for the different biomes, ores and decorations.
The patterns on the ground were made with the beach biome's vertical_blend attribute, which allows the gravel to sometimes replace the default ground blocks of the biome applied to the flat area above it. Different plants could be assigned to the different blocks, so that they are placed on these patterns when the map is built.