This has been wished for many times in the past, and it has been rejected for many reasons. However I think I have a good idea for artillery to be placed in game.
The LVT-A4 was used as an artillery piece in WW2 and is quite useful in doing so. The rounds can travel well over 6k and are very effective against town buildings and other soft targets. I was told by Ack-Ack or some other information god that this was true a while ago, so i'm going off of that.
As for normal artillery pieces, we could put the M12 GMC 155mm Mobile Artillery in game.
Specs:
Armament: 155mm gun, M1917 or M1918
Armour: 10mm - 25mm
Engine: Continental, R975-C1, 9cyl. radial, air-cooled, gas, 400hp
Speed: 24 mph
Range: 140 miles
Crew: 6
Weight: 29.5 tons

I think that adding the M12 would be a great starting point for adding artillery to the game. Although only 100 entered service they did see combat and they were used in infantry support with multiple divisions. With the current shore battery/CV gun aiming settings we could give this to the LVT-A4 and the M12 without having to insert a whole new aiming system. This would drastically cut back on the time needed to get artillery in game, thus leaving less work to do on the M12 itself. I did a quick research on this vehicle due to it being late, so don't bite my head off if a few facts are wrong.