In 1920 compared to 1919, what were the possibilities for long term peace in Europe?

It looks to me like you are asking what happened in 1919 that
made the outlook for peace worse in 1920 than it had been in 1919.  I would argue that the Treaty
of Versailles, signed in May of 1919, made the outlook for peace worse (though this is not to say
that Germany was not at fault for what was to come).  So I would answer your question by saying
that the prospects of peace were worse in 1920 than in 1919 because of the Treaty of
Versailles.

You can argue that the Treaty of Versailles reduced the
likelihood of peace because of how harshly it treated Germany.  You can say that the demand for
reparations and the taking away of German territory made it inevitable that Germany would be
unhappy and would eventually break the peace.

You can also say that
the US's failure to sign the Treaty and to join the League of Nations made peace less likely. 
With one of the most important countries not in the League, that body had much less moral
authority to stop countries from acting aggressively.