Another idea is to improve Poland's RR system. They, like most nations at that time, are very dependent upon it. While they already have armored trains, these are of less utility than AA guns on regular trains. Additional AA batteries of all calibers, or even mixed calibers, would be extremely useful in keeping trains safe from aerial attack, allowing troops and supplies to reach where they are needed. Such batteries could also be parked on sidings to act as protection for other assets.
Whilst I'm on a caffeine powered roll, co-operate more with Imperial Japan. Yes, I know they were despicable rat fuck bastards (there is far too little opprobrium heaped upon their actions leading up to WWII, but that's for another thread) BUT, they made some fine aircraft, and engines. If any of you are suitably knowledgeable of aircraft of that time (I am far too much of an amateur to add anything substantive to such a thread) and would care to see what can be made of the two countries combined efforts, it could prove quite interesting.