superflat World Y coordinate option (Up and down before generation)

Hello Mojang,

Considering the new caves and cliffs update and that super flat worlds have been lowered to naturally generate at y= -60 to -64 with 4 blocks(3 dirt, 1 bedrock), there should be an option to move the generation of the super flat world up and down the Y-axis before generating the super flat world.

Since the new super flat worlds generate at Y=-60-(-64), I can build higher but because of all the extra space, There needs to be an option or custom coordinate the super flat world up and down the Y-axis.

I would love to be able to build underneath the super flat world beneath bedrock in creative modes such as hanging cities, underground caverns, underground upsidedown adventure maps and others. If I have the option to move the generation of my flat world at y=0 or higher.

Thank you :)