There would be Five resources:
Manpower - Directly linked to population, perhaps with a 2:1 ratio or something similar. Manpower would be essential for infantry units, but a token amount would be required for just about anything.
Income - Same as it is now. This would be the primary economic resource, and would be required for all military units and construction.
Oil - Empires start generating 1 Oil per turn. Oil resources generate another 1. Refineries can boost that generation. Oil would be required for just about anything mechanized - from tanks to ships.
Steel - Empires generate 1 steel per turn. resources boost generation by 1. Factories/Metallurgies can further increase income. Steel would primarily be used on warships, but would also be a factor in some land units.
You'll notice that Oil and Steel are on an Income model in this system. Instead of acting as caps on recruitment, you gain a certain amount each turn, which you can use to build up your army, or trade/sell to other empires. This lets us have complicated resource requirements without forcing people to keep track of all these different caps; you earn it, you spend it, and you're done. Manpower and Maintenance end up being the only hard caps on construction.
So for example, a battleship might be something like this:
Recruitment: 20 Income / 1 Manpower / 5 Oil / 6 Steel
Maintenance: 5 Income
The 1 Manpower goes against your MP cap, while the 5 income is a maintenance cost. the Oil, Steel, and initial building cost are all spent at once and forgotten about. Simple.
This system would let players build up armies for engagements, then deplete them in battles and fights. Owning resources and facilities lets you recover faster instead of just having a larger army. Since Oil and Steel are gained through an Income, they can be bought, sold, and traded in order to get the units you need.
Manpower would be linked to population, so if a division of infantry gets annihilated it would cause a penalty to population. This makes Pop growth an important game feature, and encourages people to be careful with their units.
Note that this system does require each and every unit recruited to be important. If we have armies of 50+ divisions it's going to be too complicated. This is a system where every division and battleship bought would be a big investment that has to be preciously defended and watched over. I think that's a good thing, but it would require a reflection in the prices.
Personally, I really like this idea. It encourages people to trade and work their resources to build armies, which they can then expend fighting one another. Resource control becomes very important, but in a long-term sense rather than a grab-it-and-get-10-more-slots one. It would also open up a lot of possibilities with traits and such. Certain ideologies or governments could have better income or greater costs. Draft could majorly boost your manpower but hurt your unit quality. Industrialist could make steel refineries more efficient. There are so many fun options, and they could all be encompassed in an easy-to-use system of resource income, with three of the resources as static map points by region.
Ideally, we would aim for players to have around a division for every million soldiers. So an Empire with 60 million would have around 60 divisions total, maybe more or less depending on their distribution of units. This would mean that a battle would usually include perhaps a dozen or two divisions, with only Stanlingrad-sized engagements getting into the 30-40 ranges.