I proposed the idea earlier: make Luxury ressource's happiness effect not capped.
For example, the 1st amount of resource gives you +1 happiness, and it would scale like that:
1 Luxury: +1 Happiness
3 Luxuries: +2 Happiness
6 Luxuries: +3
10 : +4
15: +5
21: +6
If you get [Wonder], the increase will decrease)
For each individual resources. A worked natural spot of Gold might provide you naturally with 2 gold. But a mine will double(triple?) that amount. Technology will also, over time, increase that amount. So an initial 3 gold might yield 9 into the classical ages, and 15 in modern ages.
If you have, let's say, 18 gold, you have an excess of 3 you might want to trade to get 3 whales, which is +2 happiness. Or you can trade away the other 5 (-1Happiness) to get 3 Ivory and 1 Incense (+3 happiness).
That way, the game rewards holding a monopoly on a ressource, as you can have more complex trade agreements over small quantity of luxury ressources. But you can also want to get all the available [Luxury] around.