The most important issue when making a map/scenario/game which starts before the usual setups in A&A, is to play out different possible outcomes of the war, setting different premises than the usual ones. In the 41 setup in AA50 Germany and Russia is already at war, we cannot make other changes, then we’d make another game, not only house rules. Same goes for Germany and UK, no player can choose not having a war between UK and Germany in 41.
The point of designing a map or scenario which starts in 1939 or 1940 is to make it possible for Germany to not attack Russia, at least not until later, or to Join Russia against UK, or Germany joins UK and goes against Russia. A possible setup with mostly defensive units which makes it suicide for both Germany and Russia to attack each other in the beginning of such a scenario, they’d have to build offensive units first. If Germany went all out against UK in the real WW2 they might have taken UK out of the “game” so to speak.
Or a possible scenario in which Germany and Russia makes an alliance, and conquers all the land the can until there are only two powers left, Germany and Russia, which then goes to war to settle once and for all who shall rule the world.
US is the single most powerful nation in any scenario, at least should be, but if all other territories are captured by other powers, then US will not be the strongest anymore, at least not if two or several powers makes a join venture in the prospect of removing the US as the superpower to become, once and for all. The conquest for global supremacy can take many different directions.