I hope they build the Washington Naval Treaty into this system. Up until 1938, most of the major nations abided by the strict tonnage guidelines under the treaty, which meant each class of ship was a series of tradeoffs between speed, armor and weaponry. Those design tradeoffs affected performance in major ways, especially during the start of the war. Maybe something like a simple cap on exp. that can be used per ship until the treaty restrictions are no longer in play.
That treaty was responsible, among other things, for the Wasp being built with the shortcomings that it had. The US tried to squeeze in one more carrier under the tonnage cap and had to cut some corners to do it.