If done wisely, you need to select some groups of units of different time periods (at least a beginning, a middle and an end of war)

In Panzer Corps, most armies have a choice between 'normal' infantry and 'heavy' infantry, which are slower but have better attack values.
Adding HW infantry to the Soviet equipment would give the player a choice between 'normal' and HW infantry, adding more flavour.

Оf course, you need to ...

The few things I know already would be to add HW infantry (with those Maxim machineguns on those wheeled carriages).
Machinegun Maxim is not a separate Squad, machinegun armament system was part of the infantry (rifle company).
Maxim had a lot of weight, which created problems with mobility ...

Мotor sled:
blefru.narod(point)ru/tank/a/nkl26.htm
Сombat battalions of Мotor sled (cmsb): Scouting the terrain and the enemy, In collaboration with ski and similar units realize exploitation objective
Transportation battalions of Мotor sled (tmsb): Transported and evacuated the wounded from the ...