If there is unit among Tiger, Panther and Brummbar that fits best to be in doctrine - it would be Brummbar. It is most gimmick out of three. T3 and T4 both will have generalist and AT specialist units, which is boring but makes sense. Certainly, there would also be serious consequences of such change in terms of faction balance.
Anyway it won't happen ever, I'm not sure if it is any good, I'm putting this idea out just for the sake of discussion.
I think a dedicated AT unit will never find its place on Ostheer T4 simply because other factions get their TD much earlier. Tiger and Brumbar can fit here because they still counter infantry, you can use them where TDs are not while it is not the case with the actual Panther. You need it where TDs are.
In my opinion the problem with the actual panther is it is a high RNG unit in wrong hands, it can be devastating if its armor bounce some hits as a useless piece of shit if its doesn't.
Players are mainly looking for reliable units, something you can control. If you take a Jackson or a Firefly, if you can't totally control their Damage vs a panther, at least you can control their health. You know they're dead in 3 shots so you use them with this data in mind.
With the panther you should be controlling the dps but the .60 accuracy on the move make it harder for medium level players. And they can't control its health neither, it is supposed to be durable but a bad RNG can force you to retreat before even firing 2 rounds. For the price it has and where it stand in the Ostheer tiering, that's simply too much.
Now I'm not sure we can make it any better, because it would simply be OP in teamgame where there are much more resources available.
So there come my idea, make it a bit over the top AT dedicated tank around where the command panther is actually but limit it to 1 and doctrinal and make the tiger stock with some stats and price adjustments. A good generalist tank is what Ostheer need after all.