Most are, anyway. We would still need the 37mm..... And that would likely mean some Panzer IIIs as our next GVs, since we have three of the four guns they used.
Panzer III's used the following guns:
37mm Kw.K. 36 L / 46.5
50mm Kw.K. 38 L / 42
50mm Kw.K. 39 L / 60
75mm Kw.K. 37 L / 24
Then also in the StG III platform (same chassis):
75mm Stu.K. 37 L / 24
75mm Stu.K. 40 L / 43
75mm Stu.K. 42 L / 48
Anything to add more pieces to the puzzle would be fine by me (no more super tanks, though). Obviously, adding in more SdKfz 251 variants would be the easiest, then adding in the Panzer III chassis as a base then filling in the different guns (similar to what we have with the current Panzer IV F), then adding in the StG III with the various guns. Etc.