The Lonely Biome mod allows you to change the biome for any world to be a single biome of your choice. Use it as a challenge to survive in one element alone or simply to test out different biomes without having to explore and find them. All you have to do is change the biome ID in the config file and whatever save you open up will change any new chunks to be just that one biome. You can also create a brand new world and that will be the sole biome. But be sure to back up any saves before opening a previous world with this mod as it will change what you’ve already established in your world.

You can make your own waterworld:

Or your own snow planet:

A vast world of forests:

Or simply bring the End to your world:

You can find a list of biomes here and all you have to do is change the number in the config to the corresponding biome you want your world to be.