Terrain type similar to a super-flat world, but generating around Y=60, with a few even layers of the world material, and either no bedrock, or bedrock with random holes sandwiched between layers of said material.

But the flat terrain is already like that isn't it? I mean in rftools dimensions (you reported to the wrong mod btw)

Oops on the wrong mod bit...
And no, not like 'Flat' terrain. This would literally be only a few layers thick, and then void beneath so you could build above and below. This thought was inspired by some comments I came across regarding the Invertigo terrain.