The less predictable the better you say? Well, it is finally time for my idea of Tajikistan taking up the mantle and leading the International Workers Movement to victory!
Now, on a more serious note, this can go three ways. Either you get a gigantic North Korea, that used brute force to survive the 1990s and maybe keep much of its Eastern European sphere of influence intact, or you get a China analogue, where the Communists abandon Communism and embark on hypercapitalism. Personally I think that of the two scenarios the first one is much more probable than the second, but there could be a third, less probable one: the USSR implements meaningful reforms slowly and successfully from the very beginning,g reaching to the foundations of the Soviet State, that make it viable to today. Depending on how far back you are willing to go, that may mean changing the implementation of collectivization, killing of Stalin's ascent to the leadership, a different NEP or War Communism, avoiding the Holodomor or the Show Trials and persecution of the 1930s, etc.
Now, all these could work and produce a USSR that would be more focused on technological and cultural excellence, thus preserving the Soviet State by means of ensuring technological capability and the creation of a Soviet culture, or its promotion by the state to justify its existence, apart from its ideological reasoning. Now, on a more practical note: the Party would probably have to crack down less on dissent, maybe establish something akin to Dubček's Socialism with a Human Face, generally stop shooting other WarPac member states in the foot(for example, I seem to recall a story about how Czechoslovak engineers and city anders had designed a great subway system for Prague and brand new, innovative Wagons, but the Kremlin forced them to adapt the system and buy Moscow's carriages, things like that could be butterflied away), while at the same time enforcing strict economic modernization. Implementing measures and schemes like utilizing networks in enteral planning, funding projects like the French Minitel, using computers and digitalization, perhaps copying the Chilean ideas of Allende and computerized socialism. All these would not necessarily make socialism work, but they would probably ameliorate more direct problems, like food shortages in the end of the Soviet Union, etc.
At the same time, one would need a crackdown on other problems: the USSR needs to abandon the idea that it can have nuclear parity with the US, and spend that money elsewhere. Ideally, use it to fix crumbling infrastructure, build monuments or large public works to the effect of propaganda, or even use it to buy out sections of the population, to "bribe" them like Austria Hungary did for some time with its minorities. The Soviets would also have to find a way to stop laziness, maybe enforce a carrot/stick scheme and universalized it.
Now, another idea would be for the USSR and the CPSU to shed their rigidity on Marxism. Truly, they behaved like a church would to safeguard its religious purity: things like the Anti-Faction Ban or Democratic Centralism only prove that. Ironically, maybe a death of Lenin right after the success of the Revolution could help here, or it would be best if a different charcpacter led that October revolution. More dialogue with Eurocommunists, more dialogue with Trotskyites, an earlier version of Glasnost and Perestroika implemented more moderately and drawn out, all those would be key elements.