I think you need to double the value of all territories.
You need to make each neutral nation valuable (common, Spain’s worth at least 2 IPCs…Angola 1, etc.)
You need to get more sea zones in between the US and UK and the US an Japan (to make naval invasions/support riskier forcing the purchase of capital ships)
You need to make all starting territories worth at least 1 IPC
Some of the Pacific Island Groups should be divided into multiple territories (ie Phillipines should be at least 2 zones, solomons maybe 2 zones, etc.)
And you might want to redress the value of units as well. Certain things like Russian bombers would be more expensive then British bombers. German BBs would be more expensive then Japanese BBs, etc.
I had a few rulesets I worked with on this. None of them were fully balanced, but at least it changed the game a bit. Heck, if you view each turn as a month, we almost followed the real war once!