I really think that having some type of "supply" route being required to supply armies when they are not on their main continent or island would add a level of strategy to gameplay that cannot be overlooked. If a resource creates X number of Iron, oil or whatever each turn some of that would be consumed in producing new units and different improvements like naval shipyards steal mills factories etc. Then having an army away from the resources would require X number of food, oil, gasoline, etc that could be deducted from the initial supply and having that be required to cross by a sea route adds the very important naval and logistic aspect. You could also have a more limited supply by on transport aircraft which cannot carry as many supplies and that would make controlling the air routes also important. I feel that logistics is far too often overlooked and although it does add some more "book keeping" for the player it also effects how you wage war strategically so it is a level of complexity I urge you to include.
It might even be a good idea at looking at the resources requiring an available route to and between cities because once again it makes them a strategic factor for the human or AI player to capture or block the availability.