First this is not a "change total land to purchased land" topic. It's radically different.
The problem as mentioned in that topic is that people with furs as a bonus resource or as a trade resource can be pushed into the naval range far to quickly just through natural growth. However moving it to purchased land also has the drawback of allowing someone to easily turtle with little downside in most cases.
What I propose is to use infra as the determining factor for when you can purchase a navy and land for the ability to blockade and battle support.
Right now you cannot purchase any type of naval vessel until you reach 2k infra. My proposal would be to raise that level to 2500 and make that the entry level for the naval campaign. This is for 2 reasons. First it allows nations a bit more time to grow than 2k. Second, many nations will slide under 2k at 1999.99 infra because of the infra upkeep cost jump. While nations could still slide in under the 2500 mark, they would not be reciveing the full benifits of their infra tier which would be at 2999.99, thus it becomes a cost/benifit determination for them whether to climb all the way up or not. For many they may not. For others they may. In addition, you could not sell infra below 2k if you own naval vessels, though you could still be damaged below that level.
Now onto land. For battle support the amount of aircraft you could launch I would base upon your purchased land and for blockades it would total land.
For he former, for every 1000, you could launch an additional 5 AC up to 5* # of ACC. This has little bearing in RL - it's mostly for game balance reasons. The closest it could come to RL comparison would be simply the fact you'd have more resources to draw upon to support larger air campaigns. The reason for it is mostly an incentive beyond wonders to purchase.
For the latter the I'd raise the % from the current 1-5% to 2-9%, but give a -0.01% calculation for every 100 miles of land you own with a cap of 1% minimum for each attempt. Thus the new formula would calculate the random number and after that number is applied, subtract the percentage to the minimum cap of 1%. Thus even with 1000 day old nation with native furs that would only give you 1500 base land and -.15%, You would have to purchase land in addition to notice a difference.
