Here are some home rules.
1. Keep the supply rules unchanged, with one exception: a city that is completely surrounded (all six neighboring hexes occupied by enemy troops, whether those enemy troops are in supply or not) is considered out of supply for all purposes. No builds are possible to OoS home cities, nor can Soviet OoS factories be moved.
Comment: The rule prevents the turtling that can happen. However, certain cities (such as Leningrad and Sevastopol) cannot be surrounded in this fashion because of nearby water. This recreates the historical stubbornness of these cities. Note also that Stalingrad was never completely surrounded by the Germans, so by this rule would have always been in supply.
2. Siberian Reserves are triggered when Germany captures 20 MSP points of Russian cities. Cities whose factories have moved still count as 5 points toward this target. When SR are triggered, place a marker on the next turn. On the Russian move of that turn, place the Siberian Reserves in the box and begin the rolls as usual. If triggered prior to January 1941, place 9 when triggered and 10 more on January 1941. Place 19 if triggered Jan 41 or after. SR are cancelled if Russia collapses and are not resumed.
Comment: If Germany invaded Russia in 1940, Stalin would have released his Eastern troops then. There would have been fewer in 1940 because the Russian war machine was still building.
3. Danzig becomes German no matter how Poland falls.
Comment: Hard to imagine the Danzig Corridor remaining intact after falling into German hands.
4. Luxembourg is part of Belgium for all Diplomacy effects.
Comment: The RAW do not allow either side to persuade the Luxembourgers to join them. That's an important hex, and there are nasty diplomatic consequences for DoWing Luxembourg. At the same time, it would be too much to require a separate diplomatic effort to bring such a tiny country in on your side. So the rule here would allow either side to focus on Belgium and get the Luxembourg hex too. Note that Luxembourg is still a separate country for DoW purposes, except that the Germans can get all three countries for one cost per 17.9.