Tack # of Hearts You Had After Each Lesson
It could be an interesting feature if Duolingo showed the number of hearts you had remaining after the end of each lesson. It would allow you to go back and see which lessons you barely got through and give you a better idea of which ones you may need to revisit.
I don't think so. I usually pass the lessons on my first attempt with few or no hearts left. Whenever I pass a lesson with 3 hearts left, that is most often because I didn't pass it the first two or three or even more times, and while repeating it so many times I had time to memorize more perfectly. This means the lessons I passed with 0-1 hearts are in many cases - but not always - easier than the ones I passed with 2-3 hearts left.