Many of the 700-plus game parameters were chosen for their ability to combine in interesting ways, rather than for obvious usefulness. For instance, construction in a city can by default generate an infinite stream of units. But suppose you want to put a limit on the numbers of that type of unit? One way is to define a material that is essential for construction of that type, let the builder have an initial supply, but provide no way to get more of that material. When it runs out, no more units!
Another trick is to motivate an activity by making it a prerequisite to the basic builtin goal of defeating the other player. The age of discovery worked this way. The kings of that time weren't interested in new lands per se; they wanted exploitable possessions, that could be used to get gold, to buy armies big enough to defeat their neighbors. You could describe this situation almost exactly, by making gold a material, obtainable only by the discovery and capture of independent gold mine units, which are thinly scattered over the world and can be found only by careful exploration.
Be inventive! Studying the predefined games should suggest many tricks; the "Tricks and Techniques" section below describes even more. Be sure to document the trick carefully, or the next time you work on the game, you might break it, resulting in unhappy players wondering why their usual strategies don't work anymore.