I have a couple of gameplay improvements to suggest.
The first is that tanks need a buff badly. Tanks should have tremendous utility in warfare, but as it stands you'd be better off commissioning a fleet of 100,000 paper airplanes to give enemy soldiers paper cuts than build a fleet of tanks since paper airplanes are far less expensive to manufacture. Tanks are incredibly fragile and not at all worth the cost, and I think they should get a buff. Right now the best way to go about warfare is to just max out your infantry, aircrafts, and navy and don't waste the infrastructure or steel on tanks.
I also have a suggestion regarding limits on how many of a certain type of building you can put in a city. I understand and agree with limiting the number of a type of building you can have in a city, but this limit should increase as you buy land. I get that it's meant to incentivize building more cities, but there's no reason I should be able to have a city with 10,000 square miles of land and be limited to five freaking barracks.