Succession scenarios are interesting, but one critical question is 'Who Dies?'
Churchill, Stalin and FDR are named, but its quite possible that others die there as well; Molotov and Eden, both potential successors, are foreign ministers and could be among the victims, as well as Cordell Hull of the United States.
That question has to be addressed to get a better answer.
The Soviet Situation is going to be rough. Andrei Zhadnov might be the next Soviet Chairman, but given the nature of Stalin's death political rivals might act to grab their share. This is going to include Molotov, Beria, and owing to the situation, possibly Zhukov--while the Army has been purged and the commissar system remains in place, it is hard to think of a better time for the Army to make political demands and receive them. This is late enough in the war that the worst case--Soviet Fragmentation--isn't going to happen. But the government will only be a temporary compromise, and the Soviet Situation is going to be precarious...
Wallace is very politically left and might make the 1944 election really interesting. But he's going to be just as out to beat down Germany and Japan as FDR. The UK's doves were discredited in 1939...
The loss of Stalin might very well lead to increased Soviet-Allied participation and no cold war. The Soviets joining the Marshall plan and moderating their tone, as well as insisting on a disarmed Eastern Europe (instead of a shackled one.) The Allies probably agree to keep West Germany defanged as well...