like the title said:
with a flashcard on the screen, with the word that I just learned, I am ready to try out making new, my own, sentences using that word.

so I'd write a sentence of my own, system would check for "semantic" meaning, would propose a better way to say it in the target language.

Next I'd be prompted to change any of the words used, like changing a noun (other gender), use another pronoun, changing singular/plurals, changing verb, changing verb tense, change sentence style (affirmative, negation, question) and see the effect of this on the sentence i first wrote.

in addition, on any error, AI should explain why such is an error (refer to a paragraph in a grammar reference), and suggest/highlight the correction to be typed (typing makes you remember better the error and the right word)

so overall, this proposed feature(s) fills the missing part, the gap between "learning a lesson" (by the user, helped by readlang) and the system "teaching" the user the new target language ...

sincere regards
Hugo