Does "happen" here mean that movie work is released in public? Or is it very exceptional case to use as a meaning of "releasing new movie"?
"To happen" here just means that she wants a second movie to be made. This can also mean that she wants it to be released to the public, although it refers more generally to the movie being created. This isn't really an exceptional case that means a new movie is released, because "to happen" can describe anything that will occur. Ex: "They just got engaged." "When is the wedding going to happen?" Ex: I've been waiting for something to happen, but so far nothing has.