A lava biome. I’m the center would be an irrupted volcano. The biome color for grass would be grey. Dead bushes everywhere. Grey trees with no leaves. “Burned/charred wood” would be new. Villagers here would trade weapons and dangerous items for expensive prices. There would be rivers everywhere but they would be made of lava. Any fish would be black. Add crows to the game and this biome would have them spawning all around to remind you how sad the biome is. Sounds wacky yes but mojang would do it right I know they would lol