An idea for practice.
I would like to play with the words and sentences given in a particular lesson. I want to practice them by arranging them into a dialog that makes sense together. The dialogs could be silly, or logical, and points would be assigned after evaluating by the Duolingo team. I think this might be a fun way to practice vocabulary.
This is certainly a great way to practice, but not scalable. There are currently 29 people in Duolingo team, not all of them being language experts. Can you imagine them checking exercises of thousands of students?
There is a similar but even better system, however: users checking and helping users. Every user specifies their mother tongue and adds other languages they know or learn with their corresponding levels in their profile. You help people to learn your native language (or languages you are fluent in), and other native speakers help you with the languages you learn and practice. Such system is used on busuu.com, italki.com, lang-8.com.
It would be cool if the same practice came to Duolingo some day, but I don't think it should be a priority right now, with all those other sites serving the same purpose.
Seeing it from a Software developer's point of view, I think it is doable, the trouble is as olimo indicated it may not be scalable. One way this could be achieved is by having the system google for the entire sentence, e.g. "I dropped on the floor"(with quotes) has 482000 google results, making it possible that the sentence is correct, or has at least been used incorrectly(informally) by many people.
After that the sentence could be sent to other students to evaluate if it is truly correct by assigning points,comments, votes, or editing the sentence.