The elegant part of the solution of making the tank the transport was that it allowed you to use the unit both as infantry and as a weaker tank (with excellent close defence). If it would be switchable, you'd either use it as infantry that can move 3 hexes, or as a tank.While modding I have thought about how tank-riders could be modeled in the game, indeed some sort of switchable unit might just work?
The Soviets had so few tanks in late 1941 that 4th Mechanized Corps, attacking at Brody in the opening days of Barbarossa, by itself had more non-recon tanks than all Tank units in Kalinin Front and Western Front combined when the Soviets started their winter offensive. Including all tank types, Kalinin Front and Western Front still only had about 1/4 to 1/5 the number of tanks the Soviets could deploy in occupied Poland/the western part of the Ukraine in June, which themselves made up only part of the Soviet total (something like 1/3 to 1/4 for all tank types). Many of these broke down and never saw action against the Germans, but the same goes for the T-34's later in the war. Of course, Panzer divisions were also a shadow of their former self in tank strength by winter.
In any campaign game, decreases in tank strengths are not likely to appear. The average German core is also a complete fantasy by 1944-1945, but for them the main balance issues pop up as soon as you can buy a core composed of Tigers in late 1942. For the Soviets, they tend to be able to buy KV-1's in the final Winter War scenario or get them as a prototype. The historical reason for using fewer KV-1's, a slower speed and the same gun as a T-34, is often less relevant in PG because a KV is virtually invulnerable to most German guns whilst a T-34 can still be hit, so higher ground defence at the cost of a slightly lower speed is usually acceptable. Depending on what kind of transport is available, a greater choice in artillery tractors/heavy trucks at the start of the campaign can also make Soviet artillery faster than their historical counterparts.
Anyway, I think the main challenge will be preventing the Soviets from forming an armoured juggernaugt too early like in most PG/PG2 Soviet mods and balancing the unit selection for that. Problematic situations would be historically weak German Panzer divisions facing numerous ~12 strength KV-1's, each with a good leader.