This link -
http://forum.paradoxplaza.com/forum/showthread.php?t=347532 - contains VERY interesting ideas. Division designer, advanced equipment system and "equipment pool", spoils of war, equipment trading, separation of "men" and "material", hard techs and theory techs etc.
Additionally:
1. Airfield and dock capacity should be limited by their level. Currently as soon as you get one 1-lvl Port/airfield, you can go on rampage with good navy/airforce, which is ridiculous.
2. Air mission efficiency should be influenced by range and it should only be possible to "drop" 1-lvl ports/airfields in a province. That way fighting for seemingly "useless" provinces with good ports or airfields become much more important (North Africa, Pacific islands etc.).
3. Combat width. Enough said.
4. Dynamic combat events. They should be situation-dependant, not totally random. "Encirclement" should have much greater chance of happening for numerically superior attacker, "Breakthrough" should happen more often if you have many tanks etc.
5. Damaged ships should move at a slower rate.
6. It should be possible to split airwings into smaller organisational units. For example, you could split one airwing into two units with strength and organisation capped at 50%. It would give us much-needed flexibility in matters related to aircraft management.
7. "Loyalty" value of unit leaders. At high dissent and/or in cases of political instability (if sth like that exists in Project Blitzkrieg) unit leaders should have a chance to rebel. Useful for China, Spain, Vichy, Soviet Union...