It might be fun to tie all resources to a special bonus or building.
I'll take a crack at it.
Edit
Here is a list. I decided to gear them to the trade routes sytem.
Aside Marble's wonder production bonus, they stack with however many of the resources are in the city's territory.
This kind of thing might require taking strength from the base trade route system, and I'm worried I set some numbers too high.
Resource Bonus List:
Food Bonus Resources: +1
on outgoing
trade routes.
(Cattle, Sheep, Wheat, Deer, Fish, Bananas)
Strategic Resources: +1
on outgoing
trade routes.
Marble and Stone: +2
on outgoing
trade routes.
(Marble retains the +15%
boost to Wonders.)
Wine and Incense: +2
and +2
on internal trade routes and +2
and +2 tourism on international trade routes. Receiving civ gets +1
on the trade route.
Culinary Luxury Resources: +1
and +2
on outgoing
trade routes and +2
and +2 tourism on international trade routes. Receiving civ gets +1
on the trade route.
(Citrus, Crab, Salt, Spices, Truffles, Whales, Sugar)
Jewelry: +2
and +2
on internal trade routes and +4
on international trade routes. Receiving civ gets +2
on the trade route. Mint now enhances Copper as well.
(Gold, Silver, Copper, Gems, and Pearls)
Clothing Luxury Resources: +1
and +2
on
trade routes and +2
and +2 tourism on international trade routes. Receiving civ gets +1
on the trade route.
(Furs, Cotton, Dyes, Silk)