I havent known about this project for long, so this might already be planned, but a feature which I have found most language learning apps dont have, is customizing the order in which you learn certain vocabulary and grammar. This could be useful for people who want to use the app to learn a language they learn in school. As an example, a friend of mine wanted to use the bird to get better at english for school, but the vocabulary that they needed was taught way later in the course at a too high skill requirement for grammar.

An idea for implementing this would be that there could be standard orders made to coherently learn the language on your own, and an option to set the order of topics yourself, aswell as adding your own vocab lists. Also there should definitely be an option to skip things you already know.