The Meaning of Life, the Universe, and Everything.
There already is a biome option in customized world type, but your entire world only consists of that biome. But Why??
The purpose of this suggestion is to have an option where you can choose if your world will be 'Full': meaning your world will consist of one biome, or if your world will be 'All': meaning you will spawn in your selected biome, but the biomes are default('All') instead, even if you change the biome size.
The Full/All option will be next to the biome option and will be on "All' on default.
Note that I'm not an expert on biome and seed configurations so I do not know if it can work.