Expand the course material
I think the course material for Japanese can be richer. I know it's still before beta stage but adding some lessons about Japan's big city name/ province would be interesting.
I still wonder where the "National Holidays" skill went. It was supposed to be one of the three bonus skills.
(scroll down to the update titled "Bonus Skills")
I'm sure they are really busy just trying to get the course on the web.
When that is done: I would like the "National Holidays" skill too. I would also love a "Proverbs" skill, similar to the one that's available as Bonus material in French. (I checked the wikipedia pages - Japanese proverbs are plentiful!)
Also, wouldn't it be great if one could buy (via lingots or even real money) more kanji?
Bonus skills in incubated courses are generally not available, for some reason Duolingo hasn't allowed them to be released.
Many teams plan to include what were intended to be bonus skills in the main structure of the tree when they release the updated version of their tree.