Instead of "countering" the King Tiger via another tank, especially a JS-2 or other such thing, just approach the issue by going back down the spectrum. Dont make it an issue of "to be able to knock out the King", but rather adding another way to kill the King.
Here is what HTC is hopefully doing with regards to GV's, IMO:
Priority: Re-modeling the Panzer IV with F and H model 75mm guns available.
Secondary: Adding a British EW or MW tank such as the Cromwell (w/ choice of 6 Pdr and 75mm), or the Valentine (w/ choice of 2 Pdr or 6 Pdr); adding the StuG III and/or the Hetzer; adding the Su-100; adding in the SdKfz 251/9 (7.5cm infantry cannon, similar to LVT4 abilities), 251/22 (7.5 cm PaK40 L/46, same gun as Pzr IV H).
Tertiary: M10, M18, Type 97 Shinhoto Chi-Ha, Char-B1, Panzer III J, the M7 Priest or maybe the M4/105 (or better yet the Sexton w/ 25 Pdr). Somewhere in there I'd like to see a gv w/ a heavy mortar mounted and be able to be aimed in similar fashion to the naval guns (indirect fire).