I once saw an interesting proposal concerning the US income while still neutral.
Turn 1 = 25% income
Turn 2 = 50% income
Turn 3 = 75% income
Turn 4+ = 100% income
Whenever the US goes to war, regardless of turn #, they immediately go to 100% income plus NOs and getting their factories upgraded.
The rules say if the US hasn’t been attacked by the Collect Income phase of turn 3, they may declare war on any or all Axis powers. So that tells me that they can actually collect their full income plus NOs on turn 3. Since you don’t actually have to attack anyone when declaring war, there would really be no reason for the US not to declare war and boost their income so the Turn 3 = 75% income above is basically superfluous.
Also, I believe there was also the suggestion that both the US and USSR, while still neutral, may not develop weapons technology (however, USSR collects 100% even when neutral because they are in a more precarious position).
Another idea for tech development would be to allow the US and USSR to purchase researchers (tech rolls) while still neutral but NOT roll them. Once they are at war, then they can roll for tech with their accumulated rolls.