I am not sure If this is suggested already, but I thought I could mention it:
An offline / sync version of (parts of) the Renshuu app could be a very useful feature. I often travel on trains with no cell connection, and I wish I could use that time to crank down on my backlog in my mastery schedules. I could also see this as very useful to people on their 12 hour flights to Japan, students in schools that limit internet access during class and for people who have limited data in their mobile plans.
I realize this is not an easy fix and that Renshuu is web based, however a feasible work around could be to limit the offline access to particular areas, such as only the mastery schedule quizzes. The next time you connect to the internet with the app it could add the studied terms to be synced with the server.
This is a dream of mine, but is still pretty far off. Even if offline is limited to just the quizzes, the quiz system itself is pretty gigantic, and often relies on a lot of data lookup not just during question generation, but also in answer checking.
It is undoubtedly the thing I want to add most to renshuu, but it would be a gigantic time investment, and also the first real feature that would require major revisions to the code base, completely swapping out the Android app with a different one (the explanation behind this is lengthy), etc.
So, I hope it happens one day, but no plans at the moment.
This could be possible as a PWA, depending on how some of the data is structured. Doing so wouldn't require any custom pages for Android, and support offline web users as well.
I can maybe offer some technical support, if interested.
