Global Climates add the option when creating a new world to select one global biome for this world. For example, you could create an entire mushroom world or a plain world without any features.

Additionally, this mod allows you to choose the number of chunks to generate initially when creating a world (during the "Generating chunks" screen).

Worlds generated by the Global Climate mod will load correctly on vanilla Minecraft, but new chunks will generate depending on the seed and chunk coords rather than the selected world biome.