I have to say I’m curious what you feel is inadequate about just installing the PWA (especially if you find WaniKani’s web interface so good you just want to tweak it in places). Because I’m like 85% certain the reason there’s no native app is because they put in the effort to design the site to be a PWA as well, meaning one can “install” it just like an app from their browser.
In any case, if you’re wondering about features, I’d say it needs to at the very least do everything Smouldering Durtles can, but it sounds like with a more WK-like UI. And speaking of SD, similar to my first question, what about it did you find so inadequate you wanted to make your own app rather than making suggestions or contributions (since it is open source)?
Also will your app be open source? I have to say I’m far more likely to use an app that is than isn’t particularly because the community seems to have a 3rd party app abandonment problem. It’s great that Flaming Durtles was open source so someone could take what was there and continue it, others we weren’t so lucky. I’m not keen on using another closed source community offering.
And perhaps one of the most important requests for me (and something I was thinking about learning how to implement myself and submit a PR to SD eventually) is a loose “integration” with the Heatmap userscript. While the Reviews API endpoint is partially disabled, Heatmap (and the library script it uses, Review Cache) can only use locally saved reviews to display your review history. This means any reviews done in app would not show up on your Heatmap. I would love it if this app saved the reviews you do in the Review Cache format and modified a local file the user can easily access with it; the user could then transfer/copy that file to their main device with Heatmap and insert those reviews done on the app to the script so their browser Heatmap includes any reviews they do on the app.