If Stalin had lost the race for the leadership how would the union have progressed? Would trotsky have continued lenins NEP?
One of the hardest things to do is play “what if” with history. Harry Turtledove can probably get away with it in science fiction, but trying to assess what actually might have happened if history had taken a different course is about as productive as banging your head against the wall.
From what I understand, Trotsky understood, unlike Stalin, that the NEP was a tactical move reflecting the actual situation in Russia. The socialist revolution hadn’t spread to Europe (or at least Germany) as Lenin and many Bolsheviks had hoped. At the same time Russia was still trying hard to recover from the world war, the civil war and a foreign intervention that had wrought havoc for almost a decade.
To keep the gains of the revolution safe, a number of programs were adopted that became known as War Communism. Lenin and Trotsky understood that these were moves made to respond to the exigencies of the situation rather than an actual communist program. Stalin, on the other hand, saw them as the way the revolution was supposed to progress.
In short, then, it is entirely possible that Trotsky might have continued the NEP - and other measures of War Communism - while at the same time seeking to spread the revolution elsewhere (not at gunpoint like in Eastern Europe after the Second World War, but through international work) but with the express goal of eliminating the NEP once the international revolutionary situation had improved.
Whatever he may have done economically (always a huge guess from the viewpoint of the historian) he would likely have been little better than Stalin, politicallywise. Lenin killed Millions, Stalin killed more millions, but Trotsky was there learning from his master, too. I expect he would have kept the more brutal policies.