Okay here are some basic doctrine ideas based on the US
Combined Arms- Basically McNair’s baby. I’d say give some sort of combined arms bonus, for having, artillery, infantry, mechanized infantry, and Armor, and aircraft taking part in a battle. Also allow infantry and artillery to blitz when paired with a tank.
Superior Firepower - Use the overwhelming abundance in material and munitions to bomb the hell out of the enemy. Perhaps preventing units killed by artillery or bombers during the first cycle of combat from retaliating?
Carrier Group- Obviously this one would probably be shared with Japan and give bonuses of some kind to Carriers.
Amphibious Operations- Basically the marine national advantage from the 2004 game.
Air Superiority- Don’t know exactly how to represent this but consider events like the battle of the bulge. Once the weather cleared the American air force made it impossible for German vehicles to operate during the day. Maybe if you have more air units involved in a land combat enemy armor, artillery, and mechanized units couldn’t attack/defend during the first cycle of combat?
Aerial Convoy Defense- Would represent the use of airbases to protect convoys against subs. Basically give planes the ability to attack subs without a destroyer present.
Daytime Strategic Bombing - Something to give additional damage to American bombardments.
The British, by contrast may have a Nighttime bombing doctrine that would reduce the damage done but render the bombers immune from enemy interception.
Germany may have something like a night fighters doctrine that could undermine that advantage, but if they are using that then they aren’t getting the benefits of, say, Blitzkrieg.
These are just a few examples. Since only one Doctrine card would be allowed at a time, each country (besides China) would at least need 1 land, air, and naval doctrine to choose from. the Majors would need more, of course. That way doctrines would be a major choice. Will Germany really try to contest control of the seas and skies or will she focus on Russia. Will America focus on its air force (okay, that was technically part of the army then), its navy, or its army?
Obviously if I go this route and add doctrines as a unique card, I should probably take the suggestion of allowing people to draw one of each card type. Or maybe just let them draw five cards of any type at the start. America, for instance, wouldn’t have as many puppet cards as the Axis.
I’m still not certain if its worth trying to come up with a handful of doctrines for each country or just keep these as national advantages.