POD: Lenin recovers from strokes.
Possibly Lenin recovers only to find that half the Party has purged the other half in the power struggle, and he has to either hurriedly fall in line and become a puppet for Stalin, or openly side with either the Left (Trotsky) or the Right (Bukharin) of the party to stop Stalin. My guess would be an attempt to rally the older intelligentsia of the party, followed by a removal to a secure hospital (or a generic soviet "auto accident") and the same Stalin. If Lenin was to stay in power, he'd need to recover when there was at least one non-Stalin branch left. So he'd be pressured into following something along their interpretation- possibly no socialism in one country, possibly gentle persuasion of the peasantry with a longer period of NEP.
Of course, I do tend to socio-economic history and trends rather than great men. What am I doing answering this? At 2 in the morning?