Heya,
Every Call-In unit gets a tech requirement attached to them. From the simplest Assault Grenadier / Assault Engineer at 0CP to the high tier units like Tiger. This could be something like Tier 1 for Assault Grenadiers or Tier 2/3 for Greyhounds (LT or Cpt) and T3/4 for T34/85s or Battlephase 2 for a Command PIV. Something that simply fits the units tech level itself - not necessarily the exact building from where it would be buildable, but just the tech level. Those requirements are definitely not set in stone, but are just a first guess for the tech level and might need adapting.
Now regarding obtaining such a unit. There are two ways, either via a call-in button like it is now or to build it straight from your HQ. Building it straight from your HQ is only possible if you meet the tech requirements and has a normal build time. Calling it in via a button does not require a certain tech level, but comes at a Premium cost (15-20% of the unit).
run.
Good idea but if you think about it, taking the t34/85, you need 120 fuel for soviet tech + 280 for the call in makes 400 fuel for 2x t34/85... WTF
If you now say you could build them single in hq, with reduced cost, you still need 120 fuel for the tech and approx 130 for a t34/85 this is just game breaking.
Further on going for such a commander is combined with a risk because you will be deprived of armour until you get the CPs. The German player will have a p4 or something else out way before thoes t34/85s so for that reason.... where is the reward for that risk??
Some thing to consider...
If you say you can build them in your t0, refering to t34/85, this would mean that if you go t4 ,which represents the tech lvl, the soviet would have t4 units with t34/85 single build on the field which would mean a bigger unit variety and advantage for soviet.
So my suggestion keep it the way it is since its quite balanced now.