Hi!
A feature which i think would be cool and relatively easy to implement is a progress chart: that is, a (lesson vs time) chart that allows the user to monitor the rate at which he/she is progressing.
I was doing this in microsoft excel to see how long I am spending on each lesson and the rate at which I am learning kanjis. This will be good for someone really busy and needs to plan far ahead. I am finishing up my masters degree in engineering management, so I have to plan quite far ahead how much time each day I can spend on Japanese studying. The graph can let me know if it is taking me too long to start a new lesson and, thus, notifies me that I should do more study.
