It covers the basic hiragana and katakana which is good, but it doesn't cover voiced syllables, i.e. going from ka to ga. It would be useful having them all in one place instead using this app for the basic ones and elsewhere for the others. Would it also be worth adding the smaller versions as well? Even though they're the same, getting the size right would be important too, especially to be able to write つ and っ properly so they're differentiable for example.
Also, would be nice to see romaji added to kanji pronunciations as well, especially since it