bring back custom biomes

I remember playing minecraft growing up there was an option to where you could create your own biomes and block layers before creating a world, I believe at the time it was just for super flat worlds but I think it should be brought back to both super flat worlds and a slight customization option for regular worlds such as what kind of biomes it has in it. Building in super flat worlds now feels so limited only being able to dig a certain amount of blocks down, this would fix that.