What if someone wrote “kids” Japanese books following common folklore (not needing to make up something new) for each level (or set of levels) that utilize only the kanji that has been learned so far. You could use hiragana or okurigana for kanji that isn’t yet known.
The books could consist of an image on each page + some text.
That way we could actually use the kanji we learn, and when we level up well beyond the kanji we’ve already learned, we’d keep seeing it and wouldn’t forget it. It could also be a further point of memorization similar to the mnemonic system.
You could unlock the books as you level up as ebooks and optionally sell physical books too? it would be a great achievement item for us users and a point of pride when we see them sitting on the coffee table. A huge stack of books for all the kanji we’ve learned