How about, Stalin dies of a heart attack during WW2 and is replaced by someone more conciliatory - maybe Beria.
After WW2, the Truman administration made a surprisingly serious offer to ban all nuclear arms, and turn control of atomic energy over to an international agency. The plan foundered for a couple of reasons, at the bottom of which was the rapidly growing distrust of the US for the USSR and vice-versa. In particular, the US refused to give up its own arms until inspections were operating in other countries, and the USSR refused to agree to inspections until the US disarmed.
I've been wondering if, with a more conciliatory leader in the USSR, a compromise couldn't be struck. For instance, the US is allowed to keep a certain number of bombs as a deterrent while the program is being set up, but the number is capped at a level the Soviets can accept. Or the US is allowed to keep a certain stockpile of plutonium metal in unmachined form.
With a more conciliatory leader than Stalin, the Berlin blockade isn't going to happen. Neither will the Korean War. Beria even proposed reunifying Germany on condition it be neutralized like Austria. Maybe he even forces Mao to form a coalition government with the KMT.
Without the crises of the late 40s creating distrust, and without the atom bomb hanging over everyone's heads, there's no Cold War. The Soviet Union and the US probably don't much like each other, but they're not locked in a life-or-death struggle over the fate of the world. Circumstances later could bring about a Cold War, but we've at least got a shot at avoiding Korea, Vietnam, and all the other proxy fights of the second half of the 20th century. And if we get Mao and Chiang Kai-Shek into a coalition government, that probably eliminates most of the deaths under Mao as well. The UN might even be effective if the permanent members of the security council aren't preparing to fight an apocalypse with each other.
Finally, with atomic energy under the control of an international agency, I bet the development of atomic power is going to look very different. It could go in all kinds of ways, some good, some bad. But it will at least prevent the push for PWRs in the US as a source of plutonium for weapons, and I bet they'd love the proliferation-resistant Thorium fuel cycle.
Anyway, it's just a brain storm so it's probably not plausible, but that's my contribution.