I use Duolingo to study Italian and I think that their streak feature is very motivating. It’s a counter that keeps track of how many consecutive days you’ve reached a certain study goal. The streak resets to zero if you miss a day.
Sometimes this encourages me to find time for study on days when I’m very busy or feel tired. I don’t want to break the streak! Simple, silly, and ruthlessly effective.
Koichi has written before about the motivational power of the streak (“30 Days of Japanese Learning”, I think). So I’m surprised that there’s no feature on WaniKani like this. I would certainly find it useful.