OK. Overnight brainstorm.
Flashman’s right, minor aligned and minor neutral IPC values should be incorporated into the controlling major power’s total, as directed in OOB rules. Minor power units are fully integrated into the controlling major power’s command structure (they move together, fight together, conquer together, etc.) Thus, the minor nations’ pieces are mainly to add flavor to the game for those who want to see distinct Belgian, Portuguese, Serbian, Montenegran, Romanian, Bulgarian, Greek, Persian, and Arab troops on the board.
The twist: when minor powers lose forces, the controlling major power may, during its Purchase Units phase, purchase units for those minor nations it controls whose capital territories are uncontested up to their maximum unit allowance (2X IPC value and only one may be Art, the rest must be Inf). During the Place New Units, these minor power units are placed in their respective capital territories (which, as mentioned, is usually their only territory).
The bonus: This rule allows players to muster new units closer to the action. Let’s say Germany wiped out plucky little Belgium on turn 1. If France is able to drive Germany out of Belgium, France will be able, on a future turn, to build 3 Belgian Inf and 1 Belgian Art right on Germany’s doorstep. Similarly, if Germany overran the Netherlands, when the Allies liberate the Netherlands, the British (who had taken control of the Netherlands when the Germans attacked), could build 3 Dutch Inf and 1 Dutch Art on Germany’s northwestern border. Or even a disastrous Allied landing in Greece, which is seen off by a combined Austro-Greek army and allows Austria-Hungary to build Greek units to help drive the Italians out of the Balkans.
The caveat: A controlled minor nation’s capital territory must be uncontested at the start of the controlling major power’s turn in order to build minor power units. Only the allied major power may build minor power units. A conquering power may never build minor power units (the Germans cannot build Dutch units after conquering the Netherlands).
Control of minor powers: In my conception, the first power to ally with a minor neutral power remains in control of that minor power for the rest of the game. Thus, if the British take control of the Dutch when the Germans attack, and the French later liberate the Netherlands, the British gain the 2 IPC boost and may start churning out Dutch soldiers on their turn (since the Netherlands would be uncontested when Britain’s turn begins).
For fun: Brazil and Siam both sent small contingents to France (Brazil’s arrived after the war and the Siamese arrived in 1918). So, Brazil and Siam enter the war when the US does and each is represented by 1 Inf. Brazil is controlled by the US and and the Brazilian Inf is placed in the US, where it may be re-mobilized if lost. Siam is controlled by the British and the Siamese Inf is placed in India, where it may be re-mobilized if lost. The controlling major power pays replacement costs.