Suggestion: Add POP Quiz & Spelling Test
It would be interesting, if all of a sudden duolingo would randomly throw a pop quiz about something we learnt, or a random test based on a learnt concept. Also something which I've seen lot of users asking for is a spelling practice.
I think a test would be better as a way to assess accuracy in spelling, because duolingo doesn't ensure accuracy in a normal lesson practice , for example it always ignores when I don't put any accents, and since I'm a lazy guy I rarely put them. So I rarely remember what accents to use.
This could also be gamified, and two or more random users take the test/pop quiz simultaneously or users can choose to start competing against each, for example a couple of friends.
At the end we see the person who's mastered spelling well i.e. has the best score, and we learn to spell accurately.