If buildings can grant resources you can make the prospector, and the miller, cast a spell that creates a guild that grants a random resource. Drains gold and vanishes if the unit leaves the city. Or perhaps a system like the mercenaries where you can buy a random resource from the guild for 20 turns.
But myself would prefer if new resources were spawned by multi choice events.
"A dwarven messenger arrives at your court. It would seem that his clan has unearthed a deposit of rich iron ore somewhere inside our borders. They are willing to negotiate a deal, gold for steel, if the price is right.
1. Sell our own iron back to us? I do not think so. Dispatch scouts and find out where these dwarves are hiding! *Spawn iron resource with fort on top (even if it destroys an already present improvement). Spawn x number of hostile barbarian dwarven units in the fort*.
2. Very well. But it better be worth it. *Pay x gold. Receive iron for 10 turns.*
3. A shrewd deal, but I think I can make you one better. Your weight in gold for the location of that mine. *Pay 3x gold. Spawn iron with mine and road.*
4. Send him on his way. I have far better use for my gold. *Nothing*.
5. Thank you for bringing me this news. However, my prospectors are tracking that deposit down as we speak. *Spawn iron resource.*"
But myself would prefer if new resources were spawned by multi choice events.
"A dwarven messenger arrives at your court. It would seem that his clan has unearthed a deposit of rich iron ore somewhere inside our borders. They are willing to negotiate a deal, gold for steel, if the price is right.
1. Sell our own iron back to us? I do not think so. Dispatch scouts and find out where these dwarves are hiding! *Spawn iron resource with fort on top (even if it destroys an already present improvement). Spawn x number of hostile barbarian dwarven units in the fort*.
2. Very well. But it better be worth it. *Pay x gold. Receive iron for 10 turns.*
3. A shrewd deal, but I think I can make you one better. Your weight in gold for the location of that mine. *Pay 3x gold. Spawn iron with mine and road.*
4. Send him on his way. I have far better use for my gold. *Nothing*.
5. Thank you for bringing me this news. However, my prospectors are tracking that deposit down as we speak. *Spawn iron resource.*"