In my defense, I did a half-hearted Wish List search on this topic, and was unable to find a recent example, so…
Adding an Assault Gun / SPG - type GV might well enhance GV game play. I suggest something mid-level like a Jagdpanzer IV (75mmL70) or an SU100 (100mmL54), so that it wouldn’t need to be perked. Note that the SU100 has the same chassis as the T34, and the Jagdpanzer IV the same chassis as the Panzer IV, which might save on development time.
Strong points compared to typical tank like Panzer IV include a more effective gun, often better frontal armor, and a lower silhouette. Weak points compared to typical tank like Panzer IV include lack of a turret, so that the gun could only be trained over a limited (15 degree?) frontal arc without turning the entire vehicle. The vehicle would also be unable to fire effectively from hull-down positions.
Game play consequences of the above are that the SPG would be advantaged over a tank due to its gun/armor combination, so long as it could keep opponents in its firing arc. This would be much more practical at longer ranges and when defending. It would be less practical at shorter ranges and when attacking. In these latter circumstances; the SPG would need to worry about opponents appearing out of, or moving out of, its firing arc, and would need to move the entire vehicle to compensate. This not only would typically be vastly more cumbersome than training a turret-mounted weapon, but would tend to force the SPG to keep its engine running, thus making it difficult to detect incoming close-range opponents by sound. In fact, given how slowly most tracked vehicles turn in AH, the disadvantages of being turretless at short ranges could be so significant, that one could justify a vehicle like a Jagdpanther being un-perked.
The SPG is thus not only different in appearance from the existing tank set, but would exhibit game-play differences as well.
I would recommend against an open-topped SPG. Using such a vehicle would probably be frustrating for players, given the number of times aircraft tend to interfere in a tank battle. Although the AA vehicles are open-topped, they can at least shoot back, and are typically looking for aerial targets.