Just a suggestion
Hi. Just wanted to give a suggestion as to the Japanese course.
The suggestion is to include kanji tracing exercises to improve kanji memorization. I think that it has happened to many of us that we only remember how the kanji is read when we have it shown to us. This is bad because we also need to remember it when we write it. Another pro this would bring is learning the order of the kanji strokes, since if you plan on taking a Japanese exam of some sort you may need to know the stroke order.
I imagine integrating this into Duo may be difficult, or at least complicate things, and there are several apps already that can help. Obenkyo and Kanji Tree come to mind as free ones I have used.
Kanji almost always have a predictable pattern to write them, however, so once you know the general rules you can write them with the correct stroke order most of the time even if you've never encountered them before. If you can picture one in your head, you will also likely be able to write it. Most kanji are just a bunch of simpler kanji combined, so treating them as such can help. Mnemonics help some people. 語 is made of 言, 五, and 口, for example, so you make a story using the meanings of the kanji and its constituents. This is the basis of the Heisig method.
That said, when it comes to actually writing, it's hard to beat paper. Touch screens just don't have the same feeling, and you'd almost never write that large in reality.