If there is an EASY way for HTC to add more gv's in to AH it is by using the SDKfz 251. HTC could very easily add in these two variants and bring something NEW to AH:
SdKfz 251-9: armed with a 7.5 cm KwK37 low velocity gun use for infantry support (think same fast firing 75mm as we now have on LVT-4, a great town killer). Nicknamed "Stummel" ("stump"). This would be a land based LVT-4, basically, but faster. Reload rate would be typically be 3-4 seconds.
SdKfz 251-22: 7.5 cm PaK40 anti-tank gun. This is the same gun on the Panzer IV F/2 I believe (or H). Either way it would be a formidable anti-tank platform not to be over looked. It should have the same rate of fire as the Panzers (5-6 seconds).
SdKfz -251-21: 251/21 - Anti-aircraft variant equipped with a triple-mount of MG151 autocannon; early version being MG151/15mm cannon, later being MG151/20mm cannon. This would be a stop gap between the Wirby and M16. The 20mm guns are already in AH, HTC would simply need to model the mount and upper chassis. however, it would be very nice to get the 15mm in to the game.
So there ya have it, three new variants that would bring something not currently in AH (think equivalents), and with minimal modeling comparing to starting from square one.