Indirect Fire weapons (like these siege guns) wouldn't really be that useful. First off, hitting anything using the map and "land mode" aiming is pretty much a crap shoot. If you lay enough fire down, sure you'll hit something, but you won't have much say in what it is.
In Direct Fire confrontations, an SP 105mm gun is pretty much guaranteed to go up like a roman candle if you hit it. Sure you'll blow any vehicle apart if you get a hit, but a hit from a tank gun, or even an aircraft cannon is likely to have you pushing up daisies.
IF HTC was inclined to add a self-propelled gun to the list, I'd be much more in favor of a German 88mm gun because it can also be used as AAA base defense against bombers. The 88 was mounted on a Sdkfz half-track, and could be used in AAA, Direct Fire, and Indirect Fire modes, and for sure was one of the most historically important weapons of the war.
EagleDNY
$.02