Yeah, I actually plan to do that myself for Gunpowder, only available with something like Sulfur + whatever. Or through a guild, don't know yet. I would also like to add some resources like Ale; like a Legends resource given to the first civ that manage to build a new wonder like The Odyssey

Each library would give +1

for Legends (and maybe +10%

if it's possible, don't remember for now) and the Great Library would give +2

...
You could create a building requiring river and something else and then give a resource thanks to that.
I enhanced the resource diplomacy system, if you're interested.