Thinking in the language
I think in order to really understand the language, you should be thinking in it as well. The benefits of duo lingo could be enhanced by original writing from learners. Like learners would create their own sentences in some way (chat, mini paragraphs, etc.). Even duo lingo could offer a simple sentence (ex: French- "Je mange la pomme") and a learner could be asked to add to the sentence, like "Je ne mange pas la pomme", or "Je mange la grande pomme" or "Je mange la grande pomme, mais je prefere la petite pomme"
For non-French speakers translation "(ex: French- I eat the apple)" I simply just negated the action of the sentence in the first example ("I do not eat the apple"), added an adjective in the second ("I eat the big apple"), and added a personal opinion to the third ("I eat the big apple, but I prefer the small apple"). I think something like that, something simple, could enhance the retention of the language.
I think you are thinking of something like what LiveMocha has. At the end of each LiveMocha lesson you are given a review and then the opportunity to say (into the microphone) and then write what you now know. Native speakers read and listen and review. I didn't like having my accent reviewed over the computer though because of my poor quality microphone but I loved submitted writing exercises!
They are at least a good supplement to Duolingo for now.