Does anyone else think that there should be a sentence builder? For example, it takes the vocabulary you know, and make it in to a sentence. So they could write it in the language you are learning so you can translate it, or write it in English so you can write it in the language you are learning
Instead of them building sentences that only relate to the vocabulary your learning in a subject, they could combine vocabularies from different subjects. For example, say they only write sentences relevant to the subject animals,"Lo squalo mangia il pesce." With the sentence builder, they would create sentences that cover words from various subjects,"miei cani bevono l'acqua dal bicchiere e mangiare il cibo per cani''
Please read the comments of Luis here: http://www.duolingo.com/comment/646939 This will give you an idea about the numbers involved in a task like the one you propose.
I might be wrong, but artificial intelligence is not yet at the level required to make it completely automatic, human inspection will be necessary to make sure that there are no mistakes, and all possible translations of any sentence are considered. So even adding a few hundred more sentences, the actual number of sentences to inspect can climb up to a million. Now lets say that a person can inspect 10 sentences per minute, and that he works 8 hours per day then it will take about a year to inspect them all.
One more thing: you propose combinations of words and skills, and when one is talking about combinations, then numbers have the tendency to grow very fast. So having to inspect a few millions of sentences is more realistic.