Regardless of how resources are handled, there has to be an option for the player/civ that has none of a particular Required For Unit resource. The game simply does a lousy, lousy job of balancing resources, or for that matter giving any Civ a Starting Position that matches its UU, UA, or UB. Rome without Iron, or Mongolia without Horses, or Morocco without a Desert - none of those are fun to play, unless you have serious Masochistic tendencies.
Luckily, just about every Era or period has some unit/units that are good candidates for 0 Resource requirements. Spearmen, for instance, require very little bronze/copper: a 2 - 3 pound spearpoint, since (despite what people think they know) most of the spearman's 'armor' was leather or canvas behind a stout wooden shield (even the classical Hoplite dropped the bronze cuirass or body armor after the Persian Wars, and Alexander's pezhetairoi, or pike phalanx, wore canvas armor until after he'd conquered Persia). The amount of metal required by arrow points is even smaller, so warriors, spearmen, pikemen, and archers of all kinds are pretty resource-independent.
Later on it gets tricky. All firearms require iron, all cannon require bronze/copper or iron, and everything with an engine is going to require oil. The answer here, I think, is to change the Quantity requirement. At the beginning of the game, in Ancient through early Renaissance periods, you are outfitting individual troops with personal weapons. The amount of resource required per soldier ranges from 1 - 3 pounds (insignificant) to maybe 100 lbs for a fully equipped Roman legionnaire or medieval mail-armored knight (significant, and they should require Resources).
Later on, a rifleman may require 10 - 20 lbs of metal for his weapon and equipment, but it is insignificant compared to 2000 - 5000 TONS of iron for an Ironclad, or 3 - 5 tons of iron/bronze for a single cannon battery, both of the same period/era.
And, if we're going to require Resources to build the unit, the amount required will go up dramatically - although, we don't have to require, say, 1 pt of Iron for a Legion, and 50 pts for an Ironclad, we should use a 'sliding scale' or the later requirements are going to be astronomical! BUT some units are going to require more than one resource: Knights require both Horses and Iron, Tanks and Landships require both Iron to build AND oil to run each turn. In fact, having enough oil to run almost every flying and motorized unit in the Industrial and later eras is going to be critical.
AND it should be a limiting resource: Germany's WWII effort was strangled for lack of oil more than any other resource (except maybe Manpower and Rationality) and the Italian Fleet was virtually land-locked after 1942 for lack of fuel. You should have to gamble on fighting a war to get Oil in the twentieth century period of the game.
And there should always be the possibility of finding and losing resources. certain technologies will make any Resource 'plot' more productive. We already have Technologies and Buildings and Improvements that increases the value of Food, but Deep Mining or Steam Engine Pumps should increase the output from mineral mines, and Open Pit/Hydraulic mining will increase the output even more (with Environmental consequences, if we want to bring back Pollution Effects). Some sources of ore will 'play out' although I think, for sheer play balance, this might have to be limited. Gold and Silver should 'dry up' most often, because it is so common in history. Finding new plots of Gold or Silver should also be pretty common, especially once they become common monetary metals (look at the number of Gold/Silver strikes just in one country, the US, in one century, the 19th - I count at least 6 major ones, several of which had a major effect on the population of the nearest city or region)
While other minerals do get worked out, better technology also allows a lot of mines to be reopened, better refining allows mine debris to be reworked, and in general I don't think the player should be required to constantly worry about losing access to all of his coal, iron, oil, aluminum or cubit zirconium.