imho, the best addition right now would be a M4A3 with the 105mm howitzer. In very strict game terms, it would give us 4 options overall. The heavily armoured T-34, the well balanced Panzer IV, the perk'd Tiger I, and the M4A3 would fill the gap with a lighter armored tank with a pretty big gun. iirc, the 105mm was good even against King Tigers.
I don't agree with throwing in the JS series or anything yet. I think there's alot of great tanks, tank destroyers, self-propelled guns, and medium tanks that would go a long way to improving the ground game. Why go through the effort of making something similar to the Tiger, when there's a whole slew of untapped resources?
Now, on a quick second to digress into the realms of insanity... you can keep your JS series tanks. You can keep your Tigers & Panthers. Give me a
Maus! 128mm main gun, and 75mm "coax". No, wait. Too small. Ahh, here we go, the P1000
"Ratte" design. Two 280mm main guns taken from a Graf Spee battlecruiser turret with the center gun removed, a single 128mm gun, eight 20mm Flak 38 AA guns, and two 15mm Mauser MG 151/15 guns... Doh, still not big enough. Ahh, here we go, the P1500. 250mm front armor, 800mm super heavy mortar, with two, small, 150mm backup guns, & 2-4 U-Boat engines to drive it. Man, Grote was definately off his rocker... but those are just freakin' awesome. Thank God none were ever built.
And I checked all those numbers to make sure I didn't mistype anything. Yes, 800mm.