More information on verb + perposition
In my experience learning languages, I have found learning which preposition goes with which verb, to be one of the most daunting tasks, probably the most.
Just like in european languages, you learn a noun's gender when you learn the noun, in the same way, learning which preposition a verb takes while learning the verb turns out to be very important, in the long run. So for duolingo to stand out, it should give information on the prepositions that go with the new verb as a hint. And there should be more tests on the same. This would really take duolingo learning to a new level.