I wonder if this idea would be interesting.
It requires that you keep track of the number of turns played.
When a pro-neutral or strict-neutral is invaded: The defending player (decided by whoever is closer to that territory) multiplies the income value of the country by the turn number and builds the units desired to participate in the defense of the territory alongside the starting units posted on the board. Unspent income is kept as bonus IPCs plundered or taken by the future owner upon conversion.
When a pro-neutral is occupied during noncombat: As above, except the new owner must builds units in that territory using as much accrued income as possible, then only the fractional income that remains is sent to the capital during collect income phase.
Examples: (treating the neutral as China in terms of placing units without a factory)
Britain invades Iraq on turn 3: Italy has the closest units so the Italian player has 6 IPCs (2 IPCs times 3 turns) and builds a tank to aid the 3 default units (infantry) printed on the board. (He selects a tank, hoping to gain it later if it survives and he can take control of it when he later moves in.)
America moves into Brazil during noncombat on turn 4: He has 2IPCs times 4 turns or 8 IPCs to buy units for the Brazil garrison that he will acquire. He builds one transport off the coast and the remaining IPC is collected during the collect income phase. (This income could be convoyed alongside the normal amount eligible.)
Anzac takes Brazil on turn 6: 2 times 6=12 and Anzac decides to place a factory on Brazil which can be used the following turn.
I don’t think this would impact the game too much, maybe its too complicated.