a) something like in the Civ games, where if you control a resource, you can use it, and you need a certain number of resources for some things. So controlling that mountain region may give you x points of metal and some high-end recruitment centers need metal points to produce soldiers.
b) maybe with the new trade system, these resources could be tradeable goods. I'm really not sure about that, though.
The major issue I see with a) is that it puts small realms at a huge disadvantage compared with large realms which span across multiple resource types unless the resources had a smaller range of effects, so instead of realm wide, maybe duchy wide or just surrounding regions. The feature is probably still far away so don't really need to balance it now.
