Vocabulary With Pictures
I was thinking that maybe there could be a picture for every word so that when you go to the Vocabulary tab and click on a word you find example sentences and also the picture. For regular nouns it could just be an actual picture but for verbs Duolingo could make little icons, like the mascot, and have them doing an action (not necessarily animated but that would be awesome). I find that when I see a picture of the word it sticks in my brain a little better than just having a few pictures and many sentence structure/listening/speaking sections.
Just a thought.