Panzer IV Ausf. F: 50mm frontal hull armor, 50mm frontal turret armor, 75mm KwK 37 L'24 or KwK 40 L'43.
This would give us a Panzer IV better representative of early set-ups such as Barbarossa, and North Afrika. We could have both cannons to represent earlier variants such as the Ausf. D with reasonable accuracy, as well as the later F2 and early G models.
It would also give us an ENY 35 vehicle with a powerfull cannon which we have been lacking.
SdKfz 251/9: a 251 with a 75mm L'24 gun, the 251/21: a 251 with 3 20mm cannons for anti-aircraft duty, and the 251/22: a 251 with a Pak 40 75mm AT gun mounted on top.
Would be a great ambush weapon, due to the small size and low profile. It would also be an effective town-killer just like the M3 75mm GMC we have now. This would also give us a fast anti-aircraft vehicle, with better firepower than the M16 MGMC.