I think the reason Panthers are unused is sort of two-fold.
First off, is that Tigers and Lynx offers "more" in one package than Panthers can do.
- With Tigers you get double the firepower and more armor. Both of these wins out against other units, primarily in defense but also in offense in the late game. (Going against a defensive line made up of or reinforced with Tigers either requires Tigers in return or massive losses before punching a hole. On offense, the Tigers are slow, but their Armor/Firepower has so much momentum that an advancing Tiger army is hard to slow down or stop)
- With Lynx, you get mobility and equal firepower to Panthers. Here speed really is the key. (Preferable to go around your enemy or attack a weak spot)
The other side of this issue is the cost. Like someone already said, once you can get Panthers, you can usually also start getting Tigers - if not right away, then soon after.
When Tigers are researched, you usually end up getting either Lynx or Tigers - depending on the situation and use, rather than Panthers, since Tigers only cost a little bit more Panthers.
(I don't remember the ore cost differences, but they always seemed small to me - at least compared to the benefits the Tiger chassis offers)
So my suggestion to balancing the Panther - and make it a more useable unit(*):
- Panther: Small ore cost increase, small Armor increase (?)
- Tiger: Increase ore cost (Significantly ?), Lower speed (By 1 or 2 levels ?, Researchable upgrade? **), Small armor increase (?, researchable ?)
* These suggestions should be easy to implement, with minimal coding involved unless I'm mistaken
** As I understand, unit speed is set on specific levels. So one could lower Tiger speed by 2 as a default, then offer a research which increases speed by 1.
EDIT: Details.