the biggest problem would be to find a developer who has the right skills and experience, and who would be willing to dive into an old, dusty piece of code like that. Especially since it would probably also be a big, unpleasant job to polish the code
Yeeep. I doubt anyone would be willing to deal with this codebase unless paid properly. 
That’s also why I think trying to upgrade this codebase to API v2 isn’t a great idea. You’ll have to change plenty of things and that’ll inevitably be a pain. When I say the codebase is a time machine on GitHub, I mean it - quite a bit in there is straight from 2013. 
Now, regarding working on a new app, I’ve thought about it a few times. Even started some work. But then —don’t even remember where— I read rumors of official apps being in development, so I figured maybe I shouldn’t bother. Add to that my free time decreasing and the project died.
It’s been around a year since then, and there’s still no official apps in horizon, so maybe whoever wrote what I read was mistaken and after all, maybe it’s a decent idea to build a new app.
For those reading this and are technically inclined, I’ve been working with Flutter for the past 2 years, and it’s fantastic. So I’m thinking, if I am to work on another WaniKani app, it’s gonna be built with Flutter, for Android and iOS (and potentially Mac/Windows/Linux). It’s also much much easier/simpler/faster to build apps with than native Android.
However, I’m not confident I’d have the stamina to build a production-ready app alone, on top of my full-time dev job and other projects. I would be totally up to set up a GitHub org and work with whoever is interested though. React or React Native experience translates very well to Flutter, for example, so if you’re in that camp, are willing to learn, and have the time, you could join up, and we could have a good time.
I’ve always done my own designs, but it’s just been what I feel looks nice and is good design – no real expertise. So if we do assemble a team, and if you’re a designer who’d be willing to help out, I’d gladly welcome input from you.
So yeah… If there’s anyone interested in any way, I’m definitely down to talk.