Well, one possible plot twist could be that a seemingly weak candidate actually has some hidden super powerful jutsu. Another could be that the exam is secretly being manipulated by an enemy of the village who wants to steal the secrets of the Jounin training. Maybe there's a traitor among the examiners who is trying to sabotage the whole process.
There are several potential plot twists. Firstly, the rules of the Jounin exam could change midway through, throwing all the candidates into chaos. Secondly, a long - lost relative of a major character could show up during the exam, either to help or to cause trouble. Thirdly, a natural disaster could strike during the exam, forcing the candidates to use their skills in unexpected ways to not only pass the exam but also save lives.