Complex meta balance issues are not as simple to fix as people often think. There are many more factors to consider when looking at proposals like this.
1. Move all call-in heavy tanks back to CP-12 (with the current tech requirements). This artificially delays their arrival and was the original position for many of these units. It is no coincidence that the majority of the heavy anti-armor commander abilities are also unlocked at this command point window. This is a simple solution that does not modify the actual capabilities of these units and so may not be seen as the correct solution, but it would in my opinion benefit the meta tremendously regardless. Being a simple fix it would also be easier to determine competitive impact.
Heavy tanks at 13 CPs came way too late for teamgames, where resources are inflated and CP gain is deflated. This meant that they arrived at a time when there were already 1-2 vet 2-3 TDs around per every enemy player, making them mostly unviable to use. Moving heavies back to 12 CPs may benefit 1v1s, but it would not benefit 3v3s and 4v4s.
It'd likely be much better to do some small adjustments (to a variety of primary, but mostly secondary stats) over 1-2 patches rather than bring out the nerfhammer and slap on 3 CPs. There have been numerous examples over the past year where we've been able to keep overperforming units powerful by making smart and small adjustments to their secondary stats and availability (for example Jaeger Light Infantry) as opposed to nerfing them back into oblivion (as many players requested/demanded). It's always a delicate balancing act and usually a very small adjustment can already push something in the right direction.
2. A more radical solution, but still one that does not involve much stat changing, would be to require heavy tanks be physically built from tech structure and with a long production time. Preferably this would be done via end-tech T4 building on each side, which may have the side effect of making Ostheer T4 preferable, though it could also be argued it makes the Tiger less accessible. I would like to see this implemented at some point in the future, given how stupid it is that heavy tanks can be called in instantaneously but not very basic medium tanks or TDs, but it is a more difficult solution and thusly not one I see as realistic at this time.
A possible solution, but not very intuitive, since they'd need to have both a CP requirement, a tech requirement, and a build requirement.
1. A 75% or so decrease in TD accuracy via the target-table when being used on medium tanks via type medium_tank. Note this type is also applied to the Panther; in this case the Panther should have it replaced with the heavy_tank target type, or otherwise lose the medium_tank type, so as not to be effected by this change (given that the allied rosters have no other armored counters to the Panther)
Not a very elegant solution either. We are specifically moving
away from invisible modifiers as much as we can to make the game more straightforward to everyone. Introducing artificial accuracy modifiers (on top of the natural accuracy modifier that is target size) is not a good direction to take.