The Panzer III and IV models were obviously redundant with the Panzer III falling into obsolescence because it could not mount a long 75mm gun. It was however an otherwise more modern design than the PIV.
What if the Panzer III had been designed a bit larger? Enough so that it could grow to 30/35 tons instead of approx. 25 tons.
The Panzer IV could have been dropped and soon forgotten. It was only built by Krupp in somewhat small numbers.
The Panzer III could have filled both roles and been large enough to take a long 75mm gun and sloped armor as well once the Germans experience the T-34 scare.
The need for a Panther class vehicle would have been much reduced. A 1944 introduction (arguably as a heavy tank) would have been quite acceptable.
More importantly, it seems to me that real large scale production of a truly effective medium tank could have begun much earlier (again, not many factories built the Panzer IV). And this sloped version of the Panzer III would be competitive until the end or almost the end of the war.
Probably not a war winner, but 1942 and 1943 could have looked a lot different.