We can pretty much be sure that the only man likely to take his place, Trotsky, would have pursued a very different policy though. I don't know much bout Trotsky, but I do know he wasn't keen on Stalin's "socialism in one country" plan.
In reality, Stalin stole much of his concrete plan from the would-be challengers and rivals in the Party. Lev Trotsky is the largest contributor, but so are Nikolai Bukharin and Grigory Zinoviev. In fact, the whole idea of the Five-Year Plan for Industrial Development is a Trotskyist initiative. But you would not know it, from Stalinist propaganda. The slanderfest organized against Trotsky is thus highly ironic, claiming Trotskyism to be a social-democratic rightist deviation, whilst the propagators pursued a program that Trotsky himself dreamed up, though in a perverted and overly concentrated form. The opposition from Trotsky came, first and foremost, from his own denunciation and subsequent exile, but there were also legitimate concerns about the perversion of his plan that I have alluded to above. According to him, things were overly centralized and full of bureaucratic overlap and other nonsense, which contributed to a top-heavy bureaucratic mess that snuffed out participatory economics. He was also critical of the forced collectivization of agriculture, it being the opinion of both Trotsky and Lenin that such participation was to be purely
voluntary, though heavily propagandized in favor of. It was Stalin, by playing on the kulak bogeyman, who initially drew support to himself by calling for forced collectivization, criticizing the voluntary system as proceeding towards socialism "at a snail's pace," a criticism he later leveled on the NEP as a whole (which leads the the above). He was not specific in his criticism of the NEP, however, about what should be done instead, and really had nothing but vagaries until he had finished ruining Trotsky's reputation, after which he adopted the rough outline of the Five Year Plan, and turned Trotskyism into another vague bogeyman, claiming it to be Bonapartism and reactionary social-democracy, but never explaining
why. It should be obvious why they never explained. Similar things were done to Nikolai Bukharin, who was fearlessly and vocally critical of the forced collectivization campaign in the early 30s, and advocated a rather large de-collectivization campaign with explanations why (he was quite the interesting economist). Stalin repeated the slander campaign against this new challenger, which ended in Bukharin's execution. He then repeated the follow-up to Trotsky's denunciation, which was to adopt Bukharin's ideas somewhat, as if they had been his all along. Subsequent retreats from collectivization followed, and were added to the singular monolith of absolute Stalinist wisdom.
EDIT: So to summarize, things would have been quite different, but not wholly unrecognizable, had Stalin's wings been clipped. The biggest difference would probably have been the Purges and GULAG system, though I think much of the starvation that occurred between 1927 and 1933 might still have happened.
