when i play i often have a lot of certain ressources
(let's say 5 pigs or 10 fishes) that i can't trade away
(everybody has those !) and that i can't use. I end up
not even bothering hooking up the ressource. Even for
key ressources, almost everybody end up with one or
two. So maybe if you can make it that a given ressource
can be used by only X units/cities/pop point/...
trading (and fighting for a ressource) can be revived.
For exemple, an oil field could only "fuel" 20
tanks/ships/airplane/..; a corn fiel could only give
it's bonus to 5 cities.