Maybe not the answer for anti-tank artillery, but back in '36 someone had the idea of taking over the anti-tank part of the panzerwaffe with half-tracks armed with a relatively short 75mm L40.8 gun. The idea did not go through, the cannon was not further developed. At the end, it was only in 1941 that two copies of the Pz.Sfl. II finished in North Africa. The gun itself wasn't too impressive (for 1941) but in 1938/40 it would have been a significant improvement over the Pak 36. And it might have been small enough to cram into a PZ III or convert into an AT gun variant. In any case, it would make confusion in the classification / doctrine between PZ III / PZ IV because it would be a stronger AT than the 3.7 cm KwK 36 and more destructive than the short 7.5 cm KwK 37 L/24. If that gun had been adopted early enough and had proven successful, the question is whether they would make two (medium) tanks at all, and what the battles during Barbarros (or in N. Africa) would have looked like.