How about a POD which precedes the outbreak of the war itself- for instance, the rejection of the Sino-Soviet Treaty of Friendship, Alliance and Mutual Assistance, after negotiations reach an impasse, with Stalin and Mao refusing to agree terms (perhaps with Stalin continuing to repeatedly refuse to renegotiate the 1945 Treaty between the USSR and Republic of China, fearing that any negotiation of the older treaty would weaken other Soviet positions in the Yalta Agreement, as he'd done for the first two months of Mao Zedong's three month visit to Moscow? And with Stalin still either refusing to return the Chinese Changchun Railway and the naval ports of Dalian and Lüshun to Chinese sovereignty, or refusing Mao's demands to make significant capital investments in these properties before eventually returning them to China)? As such, without the signing of a new Sino-Soviet treaty (with the Chinese Eastern Railway in Manchuria, along with Port Arthur and Dairen, still remaining under Soviet control as a result, and the PRC in turn still refusing to recognize the independence of the Mongolian People's Republic), the Sino-Soviet split effectively begins to take place far earlier, opening the door to reproachment between China and the Western Powers far earlier, and pre-establishing the PRC as a more neutral party in the conflict, rather than Stalin's primary proxy instigators and suppliers of manpower.
If Kim Il-sung's invasion of South Korea still goes ahead (say, in the event that the South Korean 17th Regiment really did fire first as some scholars argue, with the KPA attack across the 38th parallel commencing within the hour as a response to this action), with their tanks, artillery and aircraft supplied by the Soviets, and rigorous training aided by the assigned general of the Soviet Advisory Group, it seems likely that the North Koreans'd still be fairly successful early on; but with Kim Il-sung firmly entrenched as a Soviet ally/vassal, and not as a PRC one, the Chinese would likely be far less enthusiastic about intervening. Of course, this still relies upon Truman keeping MacArthur in check, and refusing to accept MacArthur's requests to take full advantage of the UN Command's strategic momentum against the communists by extending the Korean War into China, and continuing to advance across the Sino-Korean border, with the cassus belli of destroying depots supplying the North Korean war effort. With Kim still likely to appeal to both the PRC and the Soviet Union for military intervention as the UN/US troops advance ever closer to the 38th parallel, and the risk of PRC intervention potentially taking Korea out of their sphere of influence rather than cementing their influence over it, perhaps Stalin might be more willing to threaten direct military intervention by Soviet forces themselves?
And with those stakes (along with a potential return to the UN by the Soviets, who had boycotted the Council meetings since January 1950, in protest at the Taiwanese Republic of China and not the mainland People's Republic of China holding a permanent seat in the UN Security Council- with Sino-Soviet relations having deteriorated more quickly ITTL, might they be willing to return in order to exercise their veto?), prior to the 27 September (when MacArthur received the top secret National Security Council Memorandum 81/1 from Truman, reminding him that operations north of the 38th Parallel were authorized only if "at the time of such operation there was no entry into North Korea by major Soviet or Chinese Communist forces, no announcements of intended entry, nor a threat to counter our operations militarily"), the planned UN offensive into North Korea would've been called off, rather than running the risk of kicking off WW3 over such a minor theater? Of course, given the emergency session of the Politburo convened by Stalin on the same day (27th Sept) IOTL, where he condemned the incompetence of the KPA command and held Soviet military advisers responsible for their defeat in the Second Battle of Seoul, Kim Il-sung's continued survival as the leader of North Korea's probably less likely ITTL than it was IOTL.
With MacArthur having warned in late August that Kim Il-sung would be held responsible for the KPA's atrocities, and with the increased Red Army presence in North Korea giving Stalin the perfect opportunity to do so after , he'd probably give the go-ahead for a military coup to remove Kim from power at this stage, and have the leader of the Soviet Korean faction, Ho Ka-i (aka as Alexei Ivanovich Hegai), installed in his place as the new Supreme Leader of North Korea- most likely, with Kim Il-sung either killed in the coup, or handed over to the UN to face trial for war crimes, thus sufficiently placating the Americans and satisfactorily 'ending' the conflict, with North Korea under Ho Ka-i becoming a Soviet vassal state to the same extent as Mongolia under Tsedenbal. Similarly to Tsedenbal, Ho would likely also make similar overtures to have North Korea incorporated as a constituent republic of the Soviet Union. In Mongolia, the idea was met with strenuous opposition from other MPRP members, and was subsequently abandoned; in post-war North Korea though, with far weaker opposition factions, a devastated KPA, and a far greater degree of direct control from Moscow, Stalin would be likely to get this pushed through, and in doing so, salvage a net gain for the Soviet Union in the Korean War through the effective annexation of North Korea (which could also leave the door open for the reunification of Korea when/if the collapse of the Soviet Union happens, in a similar manner to the reunification of Germany). What do you lot reckon- does it seem plausible?