My bet is on a Soviet level bomber. Given the trend over the past 1 year or so I almost think that HTC is possibly working on the Ki-61 remodel with the addition of the Ki-100 model at the same time.
Other than that, I could see the Diamler AC Mk I (armored car/scout car). This would somewhat add something new in the fact that it could run straight in to position for a shot, and then retreat straight out without turning around. The commander could pilot the vehicle with another set of controls. At 50mph+, it could prove to be a PITA with that ability. Granted, the 2 Pdr wouldn't do much but annoy a lot of tanks but it could be another bane to the M18, M3, M8, Wirby, etc. Also, there was a version with the 6 Pdr later in the war. Heck, offer it with BOTH turrets.
I could also see the addition of the British Wellington medium bomber, the French D520, the Italian Savoia-Marchetti SM-79 medium bomber, Panzer Mk III (and MANY off shoots!), or even adding in the SdKfz 251-22 (7.5cm AT gun).