I know this is a late response, but the omissions of particular units from particular sides are predicated upon two important things - maintaining side differences, and the amount of those units produced for a particular side/in use during the target time period. We don’t want the sides to play the same, nor to differ only in statistics of particular units.
That said, I think when either Finland or Italy is in, we will probably look at some possible additions to the selections of the core four. Italy and Finland are both shaping up to have wider selections of infantry, at the cost of limited armour, vehicles and assault guns.
Anyway, what happened since this poll was posted can be summed up as partial navies and less suck, two of the less popular options. We were unable to pursue single player missions because the mission editor was abandoned by the developer, and full navies probably should not have been included since we would have to create them as another game. At the moment we’re still working on the suck angle, I’m pushing on Finland, and we’re still hoping to start with the single player missions.