Der Meister von Alles
I have now "mastered" every category in German but am still a lousy translator, reader, speaker, writer of German. It may be beneficial for Duolingo to have some categories for "idiomatic phrases", "extended sentences with multiple clauses", "slang" and etc. I wrote etc. because I ran out of ideas, but I know there are more.
I think the next step is actually using the language. Going to German forums and joining the conversation, reading German news sites (e.g. der Spiegel) There is only so much you can learn from an automatic system.