MyBiomes is a mod that allows you to create custom biomes to spawn in your world. It doesn’t replace your world entirely but it will allow you to customize what shows up when you happen upon this custom Biome. The settings are all found in the world creation settings.
From there you can decide all the facets of what you want to include in this biome. You can set things like the frequency of trees and flowers, and even what are contained in the first 3 layers.
And once you’ve created a new biome, it will show up in the Biome settings for you to set the frequency of how often it will show up in your new world. You can even disable all other biomes and have this as your only biome.
So for example you could have a Sandstone surface with TNT just below it.
Or the surface could be Diamond Ore and below that is Glowstone!
You can even customize what mobs will show up in your new biome.