A few ideas/things that needs to be reworked or would add interesting depth that I thought up:
Tech teams:
Make it much more flexible, being able to improve/widen/change tech team direction and skill (for a cost and/or over time). Kinda like slider movement/EU3s national ideas (in spirit). Armaggeddon fixed it by throwing in "absorb all tech teams" but that's a rather blunt fix.
Instead tech team skill, areas and such could be determined by: IC, Talent points (more about those later). IF you somehow manage to take over the USA, hey more talent available! Your shitty skill 4 tech teams can now be improved, with some effort/spending.
To prevent every country with say, 80 IC, to have the same skill tech teams they can of course be preset for each scenario (but improvable during the scenario) to avoid that "samey" feel from EU3. Heck, there could be a whole area of tech to research to improve/change/give more options for this area. Could help simulate industrilization and make minor a bit more playable.
Talent:
Talent points could be kinda like the ones Victoria used to recruit/generate random leaders. Used for the same thing in HoI3 + Tech team improvements + Elite regiments/units. Give a bit more depth to the manpower system (China got plenty of Manpower, but maybe not as much talent generation, a Nordic country is the other way around etc etc). Being able to "train"/generate random generals would solve the whole "nah, your country CAN'T command more than 1 fleet. Ever. Yes yes you control half the world, still no-go!".
Certain zones/regions would have higher talent regen, bomb those universities to smithereens and watch the enemy get in all kinds of trouble.
Spies:
*Send spy* *failed* *wait one week* *send spy* *failed* *wait one week* *send spy* *success!* *repeat 10 + times and proceed to completely wreck the enemy country with coups/sabotage*.
It's both clumsy, micro intensive and unrealistic. Should be streamlined more, could use talent instead of money to train spies? Cost large amounts of money to use those really cool operations? The thing that works rather well is the whole "Might have 10 Battleships. 156 divisions yada yada". Maybe just stick with that and have some kind of spy construction center + techs + ministers + spy general to handle the whole thing automatically.
This is one area that could use more abstraction.
Industry:
1. Connect it more to manpower. Researching better factories / organization techs (production control etc) frees up more manpower + gives more IC. Balances low tech industrial but high population countries with the higher tech but lower populated ones. Not to say that China shouldn't have a massive manpower advantage. Just a bit more depth + balancing opportunities.
2. Have regions/zones be connected with a certain kind of IC. You might have 200 IC total, choose to dedicate 40 of that to ships, 40 to airplanes etc.
That way you can't switch instantly between ship building 100% and Airplanes/ground army. Lock ship building to coastal regions (maybe some industry techs to unlock other regions once modular construction comes around). Now your spies can find out where that drat airplane construction is concentrated and bomb it to dust! Or use troops to capture vital areas.
"Moving the industry to the interior" could now be a option for any country to switch around where their industrial centers are located and so forth.
For the average player it doesn't make much difference and those who like more depth can have all kinds of fun. Germany gears up the naval/air IC for the invasion of Britain. The British uses a desperate gamble to blow those dams/factories to slow down the germans
Have a few sliders for how much IC goes to what field and a manual option to lock regions to a certain kind of production.
3. Resource use. Right now one can easily build up giganormous stockpiles before the war and make navies pretty much useless. Suggestion: Certain units (say Battleships) would take *extra* resources to build. Your factories consume energy at a rather steady pace (depending on tech) but once you put down 10 battleships watch those metal stockpiles dwindle. That way you could balance IC/Manpower cost with resources better, make tanks pack more of a punch/take less IC to build but if you don't have those resources...tough luck. Would make the German metal trade rather important to keep going for all those tanks and make navies more usefull as resources would be consumed at a much higher pace during war. Manage to block off the allies and watch Britains options dwindle to infantry construction. Might also need some kind of resource stockpile cap depending on nr of provinces/special stockpiles that someone suggested earlier in the thread.
4. Perhaps throw in consumer goods as a tangible resource, taking up space like supplies does now to keep dissent down. Enact dissent reducing messures and watch those goods dwindle faster than when "coupons for everyone!" is in effect.
Air:
Those insanly imbalanced bombers needs to be reworked so you can't use 4-8 naval bombers to take on the entire US navy, or 20 dive bomber units to whipe out entire armies in a matter of hours. Mostly a balancing point.
Navies:
With the reworked resource system they would be of more importance and without insane air imbalances they wouldn't die so easy. Direct changes...hmm, maybe a option to upgrade older ships with new firecontrol/rebuild them to add more AA or convert them to carriers (worse than "real" carriers, but still). Maybe have less naval zones and keep it hard to detect ships at the high seas, easier near coasts? Difficult to come up with ideas.
AI:
Except "better AI!", the whole super-stacking/unlimited range things should be solved. Plays into how bad the Naval part feels when the player is the only one who have to bother with island jumping/bases.
Super-stacking...some cap on amount of divisions depending on province size?
Keep those island *takable* without resorting to nukes.
