http://ww2drawings.jexiste.fr/Files/1-Vehicles/Axis/1-Germany/02-mPanzers/PzKpfw3-4/PzKpfw3-4.htm
What if instead of the Panzer III and IV chassis the Germans, upon developing the Geschützwagen III/IV chassis, phased out their existing production of the earlier types and adopted the new, combined parts chassis that took elements of both the Pz III and IV and combined them into one standardized Panzer?
It proved to be an excellent gun platform, being used for both the Nashorn TD and Hummel SP artillery:
http://en.wikipedia.org/wiki/Nashorn
http://en.wikipedia.org/wiki/Hummel_(artillery)
Apparently it could have also turret mounted the 7.5cm L/70 tank gun, which was the same weapon as the Panther. Because it utilized existing parts, required far less retooling for a new type of chassis, and could be maintained by mechanics familiar with the existing Pz IIIs and IVs it would be relatively easy to transition to, while having greater upgrade potential than existing models, more flexibility for conversions to TDs/SP Art, and was much cheaper and easier to produce than the Panther or Tiger, while having similar production costs to the Pz IV.
Some production would be lost in the transition, but IMHO it makes more sense to switch over than stick with the aging and limited variety of models that the Germans used IOTL.
Here is a picture of what a Pz III/IV would look like:
