If he had lived longer(I honestly forget how old he was when he passed) how different would the Soviet Union have looked? Would he have handled the second world war differently, and what of the cold war after?
Lenin was entering his mid-50s when he died. So not exactly ancient, but makes it a bit of a stretch of imagine what he would have done differently in WWII, never mind the Cold War. Consider also that Lenin had been essentially out of active politics since 1922, two years before his death, because of his first stroke.
It becomes even more complicated to consider what Lenin might have done differently when we note that Lenin's previous period in charge was during the chaos and improvisation of the revolution and subsequent civil war - not a great guide as to what he would do during a period of comparitive stability. Also you might consider that from 1922 Stalin was already emerging as a prominent poltiical figure, with some backing form Lenin. It gets even messier when we get into the debate as to whether Stalinism was a continuation of Marxist-Leninism, some kind of distortion or a complete break altogether.
That's a question that can only lead to speculation. I would recommend you consult Lenin's Last Will and Testament which can be accessed on www.marxists.org and you'll notice that Lenin warns the Bolsheviks to never allow Stalin control of the Party and thus the State.
The real question isn't whether or not Lenin survives but if Stalin is prevented from seizing control from Trotsky.