I’m not sure the USA can fight effectively in both theaters of war with such little built each turn. If you ignore one theater, then Germany or Japan goes hog wild. Hell, Europe by itself was made with a 60+ IPC USA to join the war at the end of turn 3, while the Pacific by itself was made with a 60+ IPC USA to join the war at the end of turn 1,2,or 3. Why isn’t USA’s total IPCs 120ish?
I think it is because the allies have other things to help them in the global game. For instance, 18 Russian men in Soviet Far East, Indian troops helping Africa. The other thing to remember is that in the game the US can get into action a lot faster than was actually possible in the war. Some of the sea zones in both games are massive and with a naval base allow US to build one turn and attack in their next turn. Giving the US more income could unbalance how quickly they can get into battle.
As for incremented income: Perhaps you could try reducing their starting income by 20 and increase it by 5 for the first four turns. In order to keep things even though I still think that something else would need to be added. Perhaps a one time bonus to income after someone attacks them (ie: after war has been declared on them) Maybe you give them the bonus whether they declare war or not, it sounds like it could be getting complicated… What can I say I love to complicate things if it increases strategy, haha.
More simply: Perhaps just give them a reduction in income by a factor of ‘X’ and give them that money back at ‘A’ IPC’s for ‘X/A’ turns. ie: x=20 and a=5(previous example) so the US starts with 20 IPC less in income but gains 5IPC a turn for ‘X/A’ or 4 turns.
Another interesting idea would be to give the US a very small, such as 1IPC a turn income forever at the cost of reduction of ‘X’ amount at the start up. This way the are not making huge bucks to start but if the axis cannot win then eventually it would become impossible to compete with an ever increasing US income.
I am just throwing ideas out. I hope someone can refine them and make sense of them, lol.