Personally I think that arty as it is right now in game is very broken. I think that it is overpowered in the sense that everyone can use it and it is very cheap for the damage it causes. It is very difficult to destroy because it is on the opposite side of the map. For example if you are being hit by an artillery strike with all 3 guns going you probably would want to stop them from slaughtering you but, if you try to stop them you can go back and kill them but obviously not spawn camp them. There is nothing stopping the enemy from 30 seconds after you leave from getting back onto the guns.
My proposition is to add a Hummel and a Priest. They are mobile artillery guns and I think they would make the game more fun for everybody. If they do add these I think that they would significantly reduce the range of the guns to balance them. You would be forced to be close to the enemy but not too close, it would add a whole new aspect to tanking as it is right now. This would also give an incentive to change how expensive it is to spawn a tank and the commander would have to decide what type of tanks he wants if he wants medium range artillery or having a Sherman up close to the enemy supporting the infantry. This could possibly also give the Germans a little advantage because the gun of the Hummel is a lot bigger the Germans need every advantage they can get because statistically they loose almost every game.