I think a good addition to the game would be supply lines. This would reduce the health of units who are encircled or cut off from their mainland and provde an additional variable to consider when attacking or defending. this could make things a little difficult during coding because you would then have to determine how many resources each province would provide in the event a players country is scattered.
Another good addition would be fuel. This could be together with coal, gas and oil with each mechanized or naval unit using a set amount per turn which would prevent people from spamming units like fighters and ships in exchange for a more conservative and realistic approach. instead of one player spammin 40 fighters and bombers and overwhelming their opponent, the fights would be smaller and probably based more on location, like strategic points, resources, or the capital