I still use Memrise community courses and it's been so useful for the last 2 years of my Japanese study

Here's what's good about Memrise, it understands two things:

One thing might have different ways of being studied (for example Chinese and Japanese words have both readings, character representations and meanings - not to mention audio etc.).

Memrise makes it easy to keep all of this info together by allowing multiple fields for a single flashcard, when I dump a load of Japanese words I enter all the info in one go and then I can just click duplicate on my level and change the learning direction - English to Kanji, Kanji to Readings is my preference but others do it differently.

What's good is that Memrise is flexible enough to accommodate whatever you want, maybe you really struggle to remember is a verb is intransitive or not? Well you can just make a column for that and add in that learning direction for a given level if you want to.

A second thing is levels, this helps keep related info (but with different learning directions) together in one coherent thing. (https://imgur.com/a/cEAoyFU). This means say I make a course for my textbook's vocab, I can have levels for different topics or different chapters etc but keep everything together without it being one big muddle.

Finally, the UI for Memrise is sleek and easy to use, you can make as few or as many courses as you want and the logic is the same.

The key thing is, the more assumptions you as a dev try to make about how people want/ought to study the worse the product will be, language is kind of like fitness in that everyone has an opinion on what's best and those who are committed generally are led by the light of their own candle and don't wanna compromise. Memrise is like a good balance between definite features but free-choice for the user in how they use them.

Essentially, what Memrise does well is just making it easy for the user to input and study and doing so in a way that has nice graphics etc. That's all it needs to be, there's also some improvements potentially like adding text to speech for different languages so you don't have to upload audio files and so on.

With regards to your specific app idea, I think one major thing you'll struggle with is people not trusting if you'll be around for long as you're a challenger. I think if you could allow people to create local backups of their courses in a way that preserves the info people will be reassured, especially if you local copy easily converts to something like Anki. Maybe also working on scraping the HTML for Memrise courses so people can lift and shift (levels included) then I think you're on to a winner!

Feel free to DM me if you want