how would the Soviet Union had devloped if Vladimir Lenin had lived longer
Assume as a PoD that Fanny Kaplan doesn't seriously wound Lenin in 1918. This minimum PoD is that instead of shooting Lenin through the neck and left lung with one bullet, and in the left shoulder with another, Kaplan grazes Lenin's head with one bullet, and hits the flesh of his upper right arm with the other. This is just about as threatening as the OTL wounds, but Lenin recovers quickly and completely from the injuries.
Lenin might live to be 75, which would put his death in 1945.
It is not likely that Soviet internal policy would be significantly moderated.
Lenin was a fanatical Communist, who regarded wholesale terror and mass executions as necessary elements of the Revolution - for subduing and destroying its opponents.
During his remaining life OTL, however, Lenin was only
primus inter pares among the Bolsheviks. There was a leadership circle, not an autocracy. OTL the autocracy was established in the early 1930s by Stalin. It does not appear that Lenin ever sought the same degree of autocratic supremacy that Stalin wanted and got.
Nor does it appear that the ruling circle of 1924 to 1930 broke in any significant way with Lenin in policy. That is, the policies followed by the USSR then could plausibly be described as what Lenin would have advocated had he lived.
A second question arises - would Lenin, as his heirs did, have read out of the Party those Bolsheviks who opposed the policies enacted by the controlling faction?
I say probably not. So the ruling circle remains intact rather than be whittled down by Stalin.