1) Magic weightless gold. Bring back banks, and with it some good role playing. Feel free to add some inflation and basic economics as well. Spend a lot of gold? Prices go up. Major towns could be the origins of these economical hubs.
2) Merchants unlimited gold, but only this and that amount of gold "per trade". The "quality" of a trader could (also) mean how quick turnaround times he has for obtaining new currency. The best, maybe once a hour. Next best, twice a day. Medium, once a day. Below average, maybe twice a week. I also want other AI visiting the stores, maybe overhearing them conversation about quality or whatever.
I wouldn't mind fractions of gold (or weight for that matter). If I want to play a trader where I exploit traveling back and forth buying and selling wares (once skills makes it worthwhile), it shouldn't come cheaply by having fast travels.
But silver and copper? Nah, I just don't think it fits.