What is needed, as has been touched on already, is a general overhaul of ALL resources.
First, recognize that the Bonus, Strategic, and Luxury/Amenity division is utterly artificial. All resources should be identical in that you need amounts of them depending on what they are being used for, you need increasingly higher technology to find more of them, and with certain technologies some Resources can be substituted for others, and some (mostly Luxury) become Obsolete late in the game.
Example:
Copper - now a Bonus resource and so only useful to add some effect to a tile. In fact, 90 - 95% of all bronze is copper, so it is a requirement for your first really useful metal tools and weapons. And, in turn, bronze saws may be a requirement for the Wheel, since stone or copper tools do not work to saw planks for solid wooden wheels (which appear in several places around 3500 BCE, but in every instance appear where bronze tools have also appeared).
So, Copper is both a Production enhancer, a Eureka material, and a Strategic resource. As a tool material, you need even more of it than you will need for weapons, because all those bronze saws, adzes, blades, hammers, etc are going to Wear Out as you use them to chop wood, saw planks (it will provide a nice Eureka for advanced boat building also) and chisel stone into statues and pillars (and Wonders)
But later, Copper is also a requirement for electrical wiring, so will be need in even greater quantities in the late Industrial Era for Amenity uses like lights, street lights, home appliances, telephone, telegraph (and therefore, to run your Railroads) and production uses to build electric motors to run everything from Power Generation to hand tools.
Like most Resources, you will need it from one end of the game to the other, and in Quantities that will require you to keep looking for it.
Which brings up resource Depletion.
It's not at all difficult to come up with a 3-tier technology increase in Resource Exploitation. Plant resources like grains, spices, Cotton, Silk, can be increased through Irrigation, Field Rotation, Hybridization or Plant Genetics, and Mechanized Agriculture, technologies ranging in time from the Ancient/Classical Era (Irrigation) to the Modern Era (Mechanized Agriculture) and later. Deposit (metals and other ores) Resources start with Surface Deposits and Shallow Mines, then Deep Pit Mines in the Medieval Era, and finally Open Pit Mining in the Modern and Atomic Eras, which essentially can remove entire mountains or hills and create tile-wide craters that can be seen from orbit .
And also, all resources come in two flavors: Normal and Industrial Quantity. You always need Food in Industrial Quantity - even a small (10,000 people) Ancient City needs, after all, 25 - 40 tons of food a day, and Rome at its Imperial height needed a 1500 ton capacity freighter docking every other day to keep its population supplied. Before the Industrial Era, most other resources are 'Normal' - 100 pounds of spices will make you rich, 100 pounds of Iron will equip several soldiers and 100 tons of iron will arm, armor and equip and entire Roman Legion.
But, in the Industrial Era suddenly you need 100 tons of Iron to build a single kilometer of railroad track, not including the engine, cars, and other infrastructure to make it work. A single Ironclad will require several thousand tons of Iron - heck, a single Frigate carries over 100 tons of iron cannon on average. That means that the increased ability to get resources through Deep Mines (with Steam Pumps at the very beginning of the Industrial Era) will match your increased Need.
As for Strategic Requirements, Horses remain a Strategic resource until well into the Atomic Era. As noted, the German Army required horses throughout WWII. In fact, throughout the war they had more horses than motor vehicles, but only the USA and Britain of all the belligerents in WWII did not use any quantity of horses - but then, they had large truck-producing industries and more importantly, access to virtually unlimited supplies of Oil.
As a footnote, the second-larges Cavalry Force in history was in the Soviet Army, which in April 1943 had over 230,000 mounted men in its cavalry arm. Only the Mongols ever fielded any larger mounted force.
Just another note, after Oil and Horses, the two most requiredStrategic Maintenance Resources in the Modern and early Atomic Eras are Cotton and Iron. Why? Because approximately 1/4 to 1/3 of the industrial capacity of all the major combatants went to producing ammunition for all those artillery pieces, AA and AT guns, tanks, ships, and aircraft - and they used up the stuff by the 100s or 1000s of tons a day, and virtually all the explosives were based on Cotton as a major component and Steel for shell casings.
We aren't likely to see a thorough rethink of Resources in Civ VI, but the entire Resource System needs to be overhauled for Civ VII.
