I actually really like that idea. I always wanted a way to actually dig deeper in Minecraft (what a play on words) and when I saw custom flat worlds and tunneler's dream, I thought of mod that gave a you a new generation method, maybe "Layers"? it would extend the build height and generate layers like every 128 or 256 blocks, and each layer is made of tougher stone and have more valuable ores, but also more lava and more new, dangerous mobs. Using the ores you could make new tools and gear, and as you went down you'd need oxygen and a suit to protect you from the heat. Maybe add mineshaft elevators that needed rope, and easy transportation systems (maybe an actual teleporter using elements found very deep underground )
I'm sorry, but the ideas just keep coming to me lol but thanks for the amazing idea, I think an addition like that would really add the "MINE" back in "MINECRAFT"