I made Default Money Resource into a strategic resource called Minor Tribe and used it as a way to produce barbarian-flavor units in cities that are adjacent to that resource using unit-generating improvements that replace each other the way different kinds of power plants do. Some improvements produce workers (ancient slavery), some produce a strategic resource called "Slaves" (commercialized/late medieval slavery), some produce a "Mercenary" unit that can be upgraded to Swordsman, some produce a "Barbarian Conscript" unit that can be upgraded to Phalanx, etc. In the industrial era the only improvement you can build with it is "Reservation" so these barb improvements don't compete with power plants.
Btw, does anyone know how to make a tech that can be traded but not researched? I'm thinking about how to give distinct civs access to distinct organic resources (wheat, rice, potatoes, etc.) that could spread across the map once traded.