Expand the Y-axis for all dimensions into the negative range.

Negative values would stretch from Y=-1 to Y=-128. For the Overworld, Y=-124 through Y=-128 would be the new Bedrock range. Between Y=4 anf Y=-4 (including all the old Bedrock) a new stone type separates the upper world from the lower world that's 4X tougher than Obsidian to cut. (Or just make it all Obsidian.) Negative Y Overworld can be huge caves, multi-room dungeons, Creeper dungeons, and the Deep Dark. This way, Y=63 can stay sea level no matter what.