Sid Meier's Alpha Centauri

Sid Meier's Alpha Centauri is one of the best 4X games ever made. Built on a modified Civilization II engine, it takes Civilization and turns the coolness dial up to about a million (give or take a couple thousand).

If there was ever a game that should have a sequel but doesn't, this is it.

Unused Landmark
Nessus Canyon gives +1 mineral per square, but is not present on the Map of Planet or on randomly-generated maps. It can be placed using the map editor, though.

Unused Parameters
There are a few of these in the data files.