the thing is, most of the stalin policy - industrialization, collectivization, and the adoption of socialism in one country as the best way to continue the revolution - was shared by the rest of the party.
so leaving aside individual figures (while there may be some slight differences i doubt they would be significant, even when it comes to the famed purges - stalin did not create the conditions for the traitormania singlehandedly, and there were some actual german and japanese agents in the country) you have to sort out what the consequences would be when it comes to which political line would be followed.
the NEP was designed to allow a bit of breathing space, to let productive forces build themselves up for a bit (similar to the PRC's use of domestic capitalists in the 50s) in the aftermath of the civil war. in the countryside, this had the effect of empowering the wealthy farmers, the kulaks. by 1928, in some areas they were more powerful than the party.
i doubt lenin would have had a particularly different reaction to this trend than stalin, although there may have been some differences in the methods used to end the NEP and collectivize the country.
and the countryside had to be got under control somehow, because the ussr needed something for foreign export in order to build up its industrial capability - something that was necessary, as stalin noted, in order to be able to fight off the next invasion from the west. again most of the party agreed with this conclusion, again the specifics of implementation may have been different.
the question of a successor is murkier, but there are three tendencies - the bukharin emphasis on the peasantry, trotskys emphasis on foreign revolution, and stalin's socialism-in-one-country/industrialization. not to say each of these men invented those ideas, but their names are good shorthand.
if lenin has more influence in choosing his own successor, i doubt bukharin or someone of that tendency will be chosen - lenin disparaged him as being non-marxist and insufficiently rigorous. if it does happen though i would imagine the result would be a wealthier countryside, but similarly less industrialization by, say, 1936. and there's the question of kulak power in the countryside - can they be co-opted back into the party, as bukharin believed, or would their greater power necessitate a mini-civil war to restore party control to those areas?
similarly, i doubt trotsky himself would be selected, nor would someone else following the trotskyist line on foreign revolution - and of course, since the 20th century has shown us that anti-capitalist revolutions tend to take place in the periphery of the world capitalist system, this is probably a good thing for the ussr, since it wouldn't be holding its breath waiting for revolutions in the west that never came. on the peasants and industrial policy, the trotsky position was similar to the stalin position, although trotsky was less eager for any sort of class alliance with the peasantry (rather than just opposed to alliance with the kulaks).
my conclusion is that stalin or someone with similar ideas would end up at the helm. the details may change, but the broad trends - collectivization, industrialization - probably will not.