A note on gold and silver:
At the time when AoE 3 will probably be set, gold and silver values went up and down relative to one another often. Often silver was worth more than gold. So people who say we should have silver as resource added, and it should be worth less than gold are climbing the wrong tree. They are thinking about current times, not the past. In some parts of the world at that time a certain snail shell was worth more than gold or silver and was used as the main currency. Besides that, most of the economic movement at that time was bartering, exchanging goods, and not currency.
Anyway, it makes sense to have one resource standing for currency, gold works fine for that.
One resource for nourishment: food (what kind of food doesn't matter)
Resources for building/making things: wood, stone, iron (yes, I find it necessary/realistic to add a metal to the resources)
What's still really missing is textiles. They were very important at the time we are talking about (and still are, actually). But I don't see how they would have an influence on gameplay except in a purely economical sense. Or should I have to make sure that my units have enough clothes to wear? No, that would detract from playability, so I say leave it.
Adding one resource (iron) would be enough to satisfy my desire for higher complexity (I want a new challenge in a new game), but would not make the game so complex that the playability would suffer.