Grammar Review for Russian, German
I think the "vocabulary review" screens that are now popping up after Duolingo Stories are a great addition. They help to reinforce difficult words from the story.
I think that there should be a similar feature in regular Duolingo lessons for inflected languages like Russian and German. I think I could learn Russian a lot faster if a review screen reminded me that I should have used the dative case instead of the instrumental case, for example, and then reminded me of what the different word forms were.