More block-depth for flat worlds

Now, you can't imagine how many times i had to switch from a flat world to a regular world, just because i wanted to build a house with a very big basement or a big lake for my fish, and that is just really frustrating as a person who wants to keep all of my builds in one world having to create a new world just because i want to give my witch mansion a proper enchanted style basement. So if flat worlds could gk from 5 blocks deep to like, at least 30-35 blocks deep, then i would make me very happy.