Perhaps the Russians could be really tricky and use a dies engine and wide tracks to create a relatively reliable and mobile tank.
For the Soviets: Perhaps a tank with a 76 millimeter cannon that they can produce in 1941? Could they put enough sloped armor on it that it defeats most German antitank weaponry?
Hrm. Can we handwave this weapon so that it's fairly simple to mass produce? Maybe we could see the Soviets swarm west and defeat the German invasion with hordes of better tanks.