I have state my dislike of the workings of the Allies before. The key things for me were the failing to model the increasing importance of US forces altering the balance of control over the course of the war and general lack inter allied co-operaton. This is more true for the allies than the other alliances but I think there is a solution that also models the increasing control Germany assumed in the axis. Each alliance shares it's theatres. Within those theatres army groups are assigned tasks, the control of an army group is assigned to a nation, be it AI or human. This would require a periodic screen where the army groups commanders and objectives are set. At this point nations within the alliance would also be called on to asssign units to appropriate army groups. Finally the ability affect this screen would depend upon the spending of points which are aquired by meeting objectives or assigning troops to fighting fronts. So the more involved a nation the more control it has of the war effort. In this way, Australia or Canada could maintain control of their home front but forces sent overseas are likely to be under another nations command, unless they are willing to send many troops or are particularly agressive in which case they may end up in control of a more minor front, be it Italy or New Guinea. It would also model the increasing importance of the US as more commands will invariable fall to US control over time.
Such a system might be OK if you are playing one of the majors. However, I don't think the ones who like to play minors are looking to get increased cooperation within the alliance by turning control of their units over to the computer.