What are some movies that end with an unanswered question?

"Not finished yet" indeed. Although not Truffaut's intention at the time of making The 400 Blows, Antoine Doinel became his alter ego and Truffaut continued the saga in four further (largely autobiographical) features:

Antoine and Colette which was part of the 1962 anthology film L'amour à vingt ans.

Stolen Kisses (1968)

Bed and Board (1970)

Love on the Run (1979)