Just thought I would make a suggestion for a future improvement. I've been thinking about doing this tool myself, but it would be better if it was wrapped in your already great feature set.
Sooo... you already have the Kanji and word bank flashcards which is great, but these only test your basic memory. I was thinking that it would be great to challenge the student beyond just simple memorization. For example, a tool that spits out questions and those additional sentences (in the form of a question) from different lessons. That way, whenever you mark a lesson complete some sentences and vocab can be added for testing. That way you can review previous grammar and complete understanding easier.
For example, it spits out something like: (it could be simple) お元気ですか？
or more complex like 来週の金曜日、あなたは何しますか? What are you doing in the weekend?
another example, you can pose this question (Phone call) "How do you ask if Tanaka-san is there?"
So, using a combination of Japanese/English questions to challenge students to form sentences. Kanji and word flash cards are good, but you don't really go through any process to turn it into spoken Japanese. It would be great if the tool could put the students on the spot (myself included). How do you say this? Right now? Tell me in 5 seconds! It could be timed, whatever
Many cool things can be done.