One possibility would be Stalin making his personality cult even more personally-oriented. Let's say he has a son that he deems suitable to continue the Dynasty of Steel, and there you go. With its experience in WW2, it's not implausible that the USSR could have gone the way of North Korea, giving up internationalism nearly completely in favor of collective nationalism.
IMO one thing that mucked up the Soviets badly in real life after Stalin was that they actually tried to compete with the West on the same terms, that is, they actually made an effort to halfway improve the well-being of their people. North Korea did not make this mistake, and is still around today. China changed only because Mao, like Stalin, did not appoint a successor who would continue the system*, not because his power structure was in any true danger of collapse.
*= I have the idea that with these kinds of really nasty totalitarian states, once the "Big Bad" dies there is a good deal of incentive among the surviving officials to liberalize in some fashion. But if there is a clear successor, as in the case of Kim Jong-il, the system can be preserved.