There's nothing I expect from you immediately on this, but I wanted to start a conversation around supply in general.
If there's one thing my study of military history has taught me, it's that logistics is the key component in any military victory. Within logistics, supply will win or lose the battle. To that end, I have two notes that I'd like to address:
1) Supply lines are too subject to chance. I have had games where I control most of Russia, yet my units find themselves out of supply because of partisan activity. I appreciate the sentiment around partisan activity, but when I am dominating the countryside with infantry divisions, I shouldn't be seeing quite so much partisan activity. I had a game where I had an infantry division at nearly every strategic RR junction, and nearly every city yet, despite my overwhelming military presence in the countryside (and after 3 years of occupation) I was still being disrupted by partisan activity to considerable effect every 2 weeks. It just feels unrealistic that that many cells are pulling off such tremendous disruption with such frequency when under such scrutiny.
2) Supply is too easy. Despite my objections to partisan activity above, I feel that supply is somewhat of an afterthought when planning a major offensive. All I really need to do is count hexes based on terrain type to know how far I can push. As an aside, I also feel like the scorched earth policy isn't sensible. Why would I be destroying my supply lines during an offensive? That said, needing to establish a supply depot or commit supplies further ahead of time to a particular destination would increase the complexity of supply. The idea that Germany is connected instantaneously to all other regions by rail seems untenable. Another aside, rail transport should be more limited or costly.
A few ideas that I'd like to brainstorm:
1) The ability to build supply lines, both convoy and rail, would be beneficial.
2) The ability to suppress a region from insurgency seems sensible. I could see this working well by requiring a command unit to garrison a "province" and for that command unit to be enforced with requested troops (efficiency of commander lowers number of troops required) and the chance of insurgency in the region falling (perhaps not to less than 5%?) based on the garrison requirements being fulfilled. Perhaps that number of troops requested could also be inaccurate based on the experience of the commander. Consider, an inept commander says he can control a region with 30 power, but in reality he needs 50 - despite fulfilling the request the actual requirement is unfulfilled. In this way it requires resources and command to accomplish a goal, detracting from frontline efforts, while adding a level of uncertainty common to such subjective circumstances.
3) Deteriorating supply from primary sources more rapidly but allowing that to be mitigated by building and manning supply depots. The deterioration from that supply depot would be based on how full the depot is and based on use. So, for instance, let's say there's a depot supplying an offensive to Stalingrad. On the turn previous I commit 40 oil, once I have used 40 oil my units are out of supply and cannot move. This forces resources to be committed to a particular offensive ahead of time rather than drawing from a common pool. Additionally, I would need to commit my supply trucks to that depot.
I think that's enough food for thought. I do want to thank you for making a wonderful game and I hope you only take my suggestions as friendly advice on things I would appreciate in this game.