Idea: Find out how quickly you will learn using Duolingo via a prediction market that is part of Duolingo.
Background reading: http://en.wikipedia.org/wiki/Prediction_market
Most predictions would probably be made by your friends based on your Duolingo profile, Duolingo history, and other factors (e.g., how seriously do you take learning in general?).
Moreover, making predictions about other people and seeing whether they come true, besides being fun, might also help you learn something about your own attempt to learn via Duolingo.