@Der:
I would add a level of strategy, which I’m all for. Love strategy.
Why not say everyone can transfer up to 25% of their income to an ally during their purchase unit phase? The 2:1 loss of production would not motivate me to do it, ever. If I’m the UK and have 40 IPC’s, instead of sending them to Russia so they can have 2 Russian fighters, I’m going to still buy 4 UK fighters and fly them over there. But say I could just transfer 25% over there. (10 ipcs) I would do that and Russia would get the fighter faster because they could buy their own and put it out on Russia with the money, instead of waiting for UK to put out fighters, then fly them over next turn.
What about just charging a fee plus the unit cost? For example, the U.S. could build a British destroyer plus a fee of say +2/3 IPCs. The question would be where placement would be allowed. You could in the case of G40, that could be on territory adjacent to a convoy zone. In other versions it could be the nearest industrial complex.
Whether you loan IPCs or have an ally build the unit for you the principal is the same.
It may also be reasonable to implement a system where a die roll is made by the enemy to destroy the convoy shipment. Example: The U.S. builds two tanks to send to Russia. Germany then rolls two dice at 1. Germany rolls a one and a two. One tank is destroyed en route.
Either of those might work, and still be preferable to me, over a scheme that only encourages multination stacks on defense.
I’m open to finding new ways to get a real Loan phase going