Lots of good points, Stalingradski. The introduction of mech infantry in Global 1940 is indeed a good precedent, and another one would be the introduction in Battle of the Bulge of the (so far, one-time) truck unit, which could transport not just infantry but artillery too. This truck application wasn’t, of course, the same thing as the creation of a self-propelled artillery unit, but it did reflect the fact that artillery pieces which don’t have their own integral means of propulsion do need to be transported to wherever they’re needed – by ship or railroad over long distances, and by road locally. The smallest guns can be carried aboard a truck, but most artillery pieces are towed, either by a dedicated vehicle or by a more general-purpose one like a truck or a jeep.
So in the same way that the mech infantry unit in G40 combined some elements of the infantry and tank units, a new mech artillery unit could combine some elements of the artillery and tank units. Self-propelled artillery (in its various forms) was a very important development in warfare, and it was one of the reasons that WWII was so different from the trench warfare of WWI. On the Western Front in WWI, major offensives – no matter how many troops they used, and no matter how huge was the artillery barrage that preceded their advance – tended to get bogged down when the advancing infantry (who were mobile) got beyond the range of their artillery support (which was static or had limited mobility, and hence had trouble keeping up with the advance). By contrast, WWII saw the gradual perfection of the concept of the fully motorized / mechanized army, in which troops and artillery had the ability to move at the speed of tanks (and cross similar terrain, if their vehicles were tracked rather than simply wheeled), and the development of combined-arms tactics which could exploit these capabilities to full advantage.
You’re quite right that the distinctions between WWII self-propelled guns, tank destroyers, assault guns and so forth can get a bit messy. Perhaps the solution is to remember that in A&A, the OOB sculpts are used to depict very general categories of weapon systems, even though the sculpts themselves are very specific models and classes (some of which don’t even fit very well the unit type they’re used to represent). Take the G40/2 mech infantry sculpts for example: they range from fully-tracked armoured personnel carriers to half-track vehicles to four-wheeled trucks, and therefore (arguably) ranging from mechanized infantry to infantry who are simply motorized. So the new unit could be thought of as “self-propelled artillery” in a very general sense. At its broadest, it could include pretty much everything that isn’t a true tank. Dividing the concept more finely, you could have two or even three categories:
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Self-propelled artillery, in the sense of a weapon with its own integral propulsion system (usually tracks) which is primarily intended to engage non-vehicular land targets. As noted above, this would be a hybrid of the artillery and tank units.
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Tank destroyers, in the sense of a weapon with its own integral propulsion system (usually tracks) which is primarily intended to engage enemy heavy vehicles (tanks, self-propelled artillery, tank destroyers). This would basically be a variant of the tank unit rather than a hybrid.
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Mobile AAA, in the sense of an anti-aircraft gun with its own integral propulsion system. This would be a hybrid of the anti-aircraft artillery and tank units.