Why not tie different luxuries to different techs? For example, Gems could require Iron Working, since you could say it is an offshoot of iron production. Spices could come with Navigation, since that was the primary goal of naval exploration in the middle ages. An example list:
Gems: Iron Working
Spices: Navigation (or Map Making, to make them all AA techs)
Ivory: Mathematics (Statue of Zeus)
Furs: Warrior Code (Needed archers to hunt the animals)
Wines: Pottery (amphorae)
Incense: Mysticism
Silk: The Wheel (spinning wheel)
Dyes: Alphabet (Needed ink to write with)
As you can see, I have them all AA techs, which doesn't heavily unbalance the game.
Of course, there's other ways to make interesting mods. Why not require certain units, buildings, and Wonders require other buildings? For example, the Great Library should require the city to have a normal library in it (Yes, this prevents pre-builds). Same with JS Bach's Cathedral and Sistine Chapel, they should both require Cathedrals. Smith's Trading Company should require a bank, Great Lighthouse should require a harbor, Oracle should require a temple, etc. Another possible list:
Great Lighthouse: Harbor
Great Library: Library
Oracle: Temple
Sun Tzu: Barracks
Sistine Chapel: Cathedral
Magellen's Voyage: Harbor
Shakespeare's Theatre: Colosseum
JS Bachs: Cathedral
Newton's University: University
Smith's Trading Company: Bank
Hoover Dam: Hydro Plant
SETI: Research Lab
Temple of Artemis: Temple
Secret Police HQ: Police Station
I realize it's not much of a mod, but it's enough to change certain behavior patterns, such as pre-builds.