First, let me say this is my first post, though I have been playing and lurking for quite some time.
I played a game some time ago called Aegean Strike. In it, units like engineers, artillery, air defense, etc. were used as force multipliers.
If we are to buy Shaka's arguments, and I do, we are not compelled to forbid stacking using the above types of units.
We are not allowed to stack corps, armies, and tank groups, but we could stack special units that gave the principal unit a special ability or simply added to the unit's combat power.
Of course, this would imply the introduction of these units. This could, for example, accomodate people's desire for paratroops. A paratroop corp would be a national asset and would act as a significant force multiplier when added to a army or corp stack. If the para unit were sufficiently expensive, it would be rare on the battlefield (read, historically accurate), but nevertheless used when warranted (Market Garden, Normandy).
Just some thoughts