An idea I had a while back for improving Japan's armored forces was to have them adopt the Jagdpanzer or StuG anti-tank concept from the Germans, a tank they could convert about this time in this TL is the Chi-HA by removing its turret and adding the Type-90 75mm cannon.
With a sloped glacis plate in the front, this Hetzer-like AFV would'nt be equal to the T-34 but would be better than anything the Japanese had IOTL and in the hands of well trained crews (trained by the Germans maybe?) would give the T-34 a run for its money and would lead to better designs in the future.
Another idea I had was that if the Butterflies in North Africa could lead the Germans to develop the JU-87 tank buster a couple of years earlier, the Germans could give this idea to the Japanese and they could do something similar with the Mitsubushi - Ki-51-Sonia.
I think these two adaptions are easily within Japanese capabilities at this time and would help to give their army a bit more of a bigger punch against the Soviets and would have a surprise benefit since the Soviets had already trounced the Japanese earlier in 1939 and won't be expecting the Japanese to have come with not one but two new anti-tank weapons systems.