Suggestion: List of new vocab words for all lessons.
I think it would be a good idea for each topic to have a new thumbnail indicating a list of new vocabulary words (with or without the translation of all lessons). It would allow more advance people to see if they know all the vocab and allow them to skip the lesson without having to do all lessons to make sure they know everything.
But that's not the point. The point is to know the new vocab before you try the test.
For example, I am a Intermediate German speaker, but because the process of Duolingo is kinda slow, I cannot see if there is new vocab in the lessons I do, and because I don't want to miss anything, I do all lessons even if there may be only one word I don't know.