I think the Panther D should be added to the RESEARCH TREE. It was the first Panther model of the new generation of German vehicles. They were first used at the massive battle of Kursk 1943 and deemed vital to the whole operation.
It’s a neat historical novelty just like all the other variants of vehicle in game. Just like when the Tiger H1, the first Tiger, was added
I think this would be a very simple vehicle to add to the research tree any given update