S!
I had a chance to try out Warbirds III, and while I consider that Sim to be inferior to AH, there were a few features modelled which would work well in AH.
One of them was the self propelled 75mm Artillery Halftrack. This was able to do indirect artillery barrages as well as direct fire. You could lay down barrages on distant enemy targets as long as you had a spotter present.
Now WB III picked the wrong vehicle, but there is no reason for AH to do the same. The 75mm Halftrack was not a particularly effective weapon.
More useful and I believe more common, was the the M7 Priest, with the standard U.S. Army 105mm field piece mounted in a tank chassis. These equipped all the U.S. Armoured divisions, as well as British and Canadian units. They were used in both the direct and indirect fire role. One of these firing on an airfield or other land target would make very short work of it. Over 3,000 M7's were produced.
The possibility of doing indirect fire would add another dimension to AH. A player would be able to hang back with a couple M7's, while a Halftrack or Scout Car got in position to observe the fall of shot. Additionally, it might open the possibility of bringing into the game, light observation aircraft like the Fiesler Storch.
The Geman alternative to the M7 would be the WESPE (Wasp) SP 105mm Gun, a very similar piece of equipment which equipped S.S. and Werhmacht Panzer divisions. It wasn't produced in the same numbers however.
The Soviet SP Guns such as the SU 76, SU 152, etc. were formidable in the direct fire role, however they were not generally used to provide indirect fire, being restricted by training and equipment.
