Suggestion: Reverse word count
I am doing En->Es and En->Fr. Even though I am supposed to know English to take those courses, I am not a native and I am improving a lot my English.
Right now the word count is by course, not by language, but I wonder how cool it would be if my word count for English would increase while I take a course in English.
Word count is my favourite feature, it saddens me so little attention is paid to it (word count is so underdeveloped...), you can not use skill points or lingots to talk with friends.
I think that this could be fun if implemented properly. Right now I'm taking English to Spanish, Spanish to English, and Spanish to French, and every course gives me a few Spanish words that the others don't (Well, as far as I can tell. I'm only finished the tree for English to Spanish). It would be fun to see how many words I've learned on Duolingo from all of the courses.