Kanji study cards and courses.
I'm not sure if it's integrated or not in the Japanese course, but having Kanji cards as you are going along in the course would be a huge help for many people. In many situations, you are given a Kanji word and have to figure out the Hiragana and English meaning when doing a skill.
That is fine and all, but many of us including myself struggle to remember the meaning of a Kanji after awhile. I could use study cards to improve on this, however it would be much more efficient and easier if doulingo had separate skills in the main Japanese course.
For instance, in skill level one you are given separate skills to study on. These skills are strictly Kanji and consists of just the basics, such as kindergarten and First grade level words. You must pass these skills in order to advance to the next level. Same in other levels, but gets progressively more advanced. Also, having a Kanji practice next to the regular practice would be a HUGE help in learning Japanese.
I know this is a request for Duolingo, and I think it would strengthen Duolingo a lot to have a built-in system for teaching writing systems, but
I would really suggest outside resources for learning kanji. Unfortunately, these tend to end up costing some money, but if you're serious about Japanese I think it would probably be worthwhile. There's a book called "Remembering the Kanji," I think, which is pretty popular, and a similar website wanikani.com, as well as plenty of other websites with their own methods. There are also apps for it, which will often involve writing practice. Different people feel differently about the value of hand-writing kanji today, but it might help. Websites and apps tend to at least have free trials to see if it works for you.