The new skill leveling and Japanese
As many people probably know, Japanese is really hard to learn, and even harder to teach (especially online). However, I think this new skill leveling system would work great in remedying by introducing a concept early on in the the skill, like the copula です, and later on in the skill show how the word conjugates and works in different situations.
This could work to show particles, adjectives, sentence structure, And even help teach kanji. Only problem is to make this happen, the skill tree would need to be majorly revamped.
P.S. there is this guy named Tae Kim who has this whole Japanese grammar guide, and teaches in this same way. Only problem is his site is a blog, and he is unable to make an interactive learning experience, so you have to read it. If Duolingo reached out to him, and he was willing, he could probably greatly help in improving the course.
P.P.S. I'm not saying the Duolingo Japanese course is bad in the post script above, it's actually done quite well. Just a few things they could polish up on before they leave Beta.