Making translations very compelling to learners via vocabulary building statistics.
Duolingo could show the user the number of distinct words he/she has seen while performing translations along with the number that he/she probably still remembers (because no peek was performed in subsequent translation(s) especially ones performed quite a bit later).
Something similar to what Khan Academy has? http://www.youtube.com/watch?v=hw5k98GV7po#t=04m28s And why stop at vocabulary. They should evolve the "daily progress" statistics we currently have. Everything is measurable here, error % (if you improve or get worse), amount of new vocabulary this week, which language you excel at etc.. limitless! Honestly I wouldn't be surprised if Duolingo already has this in mind. :D
As an addition to my earlier comment, it probably won't be long until Duolingo will be merged with today's education in schools. Teachers will have their own account for their classes and will be able to track their students' progress, see where each individual is and EXACTLY what they need to train more on.. something that is really hard to keep track on today.