What about things that would be fairly easy to implement early in development but would get harder and harder as development continues?
One example would be Open Cubic Chunks compatibility, which would only take an hour or two to add right now but would take days of work to add once most of the world-gen code was written. The argument for adding it would be that it would allow better mountains, allow structures such as Minas Tirith (300 blocks tall) or Barad-Dûr (several times that) to be built to scale, and allow underground locations such as Utumno or (potentially) the Roots of the Mountains to be literally under Middle Earth instead of as separate dimensions.