Super Rare Sub-Biomes should have Super Rare Items

For instance, the Modified Jungle Edge, the rarest biome in the game so far, should have a flower that grows ONLY there. It would be collectable, of course, but you can only grow new ones in that area, or another Modified Jungle Edge. Some sub-biome types are distinct enough to warrant such a feature, I think. It turns the whole world into a scavenger hunt, and if you get all X of them, you get the achievement "Botanist" or "The Collector" or something. It would warrant travelling thousands, tens of or even hundreds of thousands of blocks to find, collect and grow more of these plants or items. Id be interested to see other peoples ideas for other special biomes.