Personalized List of Mistakes
I think it would be amazing to see a list of your mistakes. Every time you get a word wrong, it's added to a list of mistakes and it tallies them. Then, you could order them by the number of times you got them wrong and see which words you struggle with the most.
I already use a pen and paper to keep track of words I repeatedly get wrong, then go back over those to memorize. But I think if DuoLingo could do it automatically, it would be a really beneficial feature.
It would also be really great (albeit hard to program) for DuoLingo to record your spelling when you make a spelling mistake. So, for example, when I first learned bird>pájaro, I kept getting it wrong and it took several mistakes to realize I kept spelling it "párajo." I think it would be interesting to see how you've been spelling words by mistake to visualize what you are doing wrong.
Just an idea!