You know what would be great? If Duolingo had games. For practice. Games would be an excellent addition. And it would help learners to remember subjects better and more interactively.
Good idea, but I would imagine a "correction" page too. We can build our own sentences in the target language and some natives correct them. We could learn from it a lot. It would be awesome. :)
Games would be mad awesome. Especially if you can earn achievements from those, too!