Hi!
Some time ago I got inspired by OS X mail app where you could use two fingers on touchpad and swipe mail to side to move it to archive. I thought that having gesture like that on WaniKani during doing your lessons for navigation could be awesome, so I implemented it. Today I discovered many user scripts from WK community which I enjoy a lot, so I decided to share my little script with you 
Here you are: https://greasyfork.org/en/scripts/24424-lessons-touchpad-swipe-navigation
Note:
Touchpad handling in browser is a big pain. One quick swipe on touchpad causes dozens (or even hundreds) scroll events which are emitted for about second or two. That’s why I implemented the cooldown before swiping (of about 2 seconds) - quick, simple and ugly solution. I think that 2 secs is quite rational time (who memorizes mnemonics faster?).
If someone wants to improve this script, I’d be ultra happy!
Regards,
mrowqa