Not sure how useful it might be, but here it goes. It actually uses WaniKani API, so it’s an API based userscript for an API based third party app.
- Create notes
- Alter questions
- Show WaniKani vocabulary details and statistics
- Warn on a wrong vocabulary kanji reading (or accept any)
- Warn after N repeated mistakes
- Lazy urls to WaniKani/Jisho/Eijiro
Keep in mind
- Everything stored in localStorage (your browser memory), it will be erased if you decide to clear browsing data. Though a backup feature will be added shortly.